schemes: fix a broken check for drive letter conflicts

Flagged by pytype locally.  It appears to have regressed in 1863584f2fba (not
yet released).

This seems like an obvious typo- `dict.isalpha()` is nonsense.  There's no crash
though because `schemes` is pre-populated with 5 schemes (that are all now
defunct), so the length of the dict is never 1, so it's impossible to abort.

hgext/schemes.py

@@ -1,175 +1,175
 # Copyright 2009, Alexander Solovyov <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 """extend schemes with shortcuts to repository swarms
 
 This extension allows you to specify shortcuts for parent URLs with a
 lot of repositories to act like a scheme, for example::
 
   [schemes]
   py = http://code.python.org/hg/
 
 After that you can use it like::
 
   hg clone py://trunk/
 
 Additionally there is support for some more complex schemas, for
 example used by Google Code::
 
   [schemes]
   gcode = http://{1}.googlecode.com/hg/
 
 The syntax is taken from Mercurial templates, and you have unlimited
 number of variables, starting with ``{1}`` and continuing with
 ``{2}``, ``{3}`` and so on. This variables will receive parts of URL
 supplied, split by ``/``. Anything not specified as ``{part}`` will be
 just appended to an URL.
 
 For convenience, the extension adds these schemes by default::
 
   [schemes]
   py = http://hg.python.org/
   bb = https://bitbucket.org/
   bb+ssh = ssh://[e-mail]/
   gcode = https://{1}.googlecode.com/hg/
   kiln = https://{1}.kilnhg.com/Repo/
 
 You can override a predefined scheme by defining a new scheme with the
 same name.
 """
 
 import os
 import re
 
 from mercurial.i18n import _
 from mercurial import (
     error,
     extensions,
     hg,
     pycompat,
     registrar,
     templater,
 )
 from mercurial.utils import (
     urlutil,
 )
 
 cmdtable = {}
 command = registrar.command(cmdtable)
 # Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
 # extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
 # be specifying the version(s) of Mercurial they are tested with, or
 # leave the attribute unspecified.
 testedwith = b'ships-with-hg-core'
 
 _partre = re.compile(br'{(\d+)\}')
 
 
 class ShortRepository:
     def __init__(self, url, scheme, templater):
         self.scheme = scheme
         self.templater = templater
         self.url = url
         try:
             self.parts = max(map(int, _partre.findall(self.url)))
         except ValueError:
             self.parts = 0
 
     def __repr__(self):
         return b'<ShortRepository: %s>' % self.scheme
 
     def make_peer(self, ui, path, *args, **kwargs):
         new_url = self.resolve(path.rawloc)
         path = path.copy(new_raw_location=new_url)
         cls = hg.peer_schemes.get(path.url.scheme)
         if cls is not None:
             return cls.make_peer(ui, path, *args, **kwargs)
         return None
 
     def instance(self, ui, url, create, intents=None, createopts=None):
         url = self.resolve(url)
         u = urlutil.url(url)
         scheme = u.scheme or b'file'
         if scheme in hg.peer_schemes:
             cls = hg.peer_schemes[scheme]
         elif scheme in hg.repo_schemes:
             cls = hg.repo_schemes[scheme]
         else:
             cls = hg.LocalFactory
         return cls.instance(
             ui, url, create, intents=intents, createopts=createopts
         )
 
     def resolve(self, url):
         # Should this use the urlutil.url class, or is manual parsing better?
         try:
             url = url.split(b'://', 1)[1]
         except IndexError:
             raise error.Abort(_(b"no '://' in scheme url '%s'") % url)
         parts = url.split(b'/', self.parts)
         if len(parts) > self.parts:
             tail = parts[-1]
             parts = parts[:-1]
         else:
             tail = b''
         context = {b'%d' % (i + 1): v for i, v in enumerate(parts)}
         return b''.join(self.templater.process(self.url, context)) + tail
 
 
 def hasdriveletter(orig, path):
     if path:
         for scheme in schemes:
             if path.startswith(scheme + b':'):
                 return False
     return orig(path)
 
 
 schemes = {
     b'py': b'http://hg.python.org/',
     b'bb': b'https://bitbucket.org/',
     b'bb+ssh': b'ssh://[e-mail]/',
     b'gcode': b'https://{1}.googlecode.com/hg/',
     b'kiln': b'https://{1}.kilnhg.com/Repo/',
 }
 
 
-def _check_drive_letter(scheme):
+def _check_drive_letter(scheme: bytes) -> None:
     """check if a scheme conflict with a Windows drive letter"""
     if (
         pycompat.iswindows
         and len(scheme) == 1
         and scheme.isalpha()
         and os.path.exists(b'%s:\\' % scheme)
     ):
         msg = _(b'custom scheme %s:// conflicts with drive letter %s:\\\n')
         msg %= (scheme, scheme.upper())
         raise error.Abort(msg)
 
 
 def extsetup(ui):
     schemes.update(dict(ui.configitems(b'schemes')))
     t = templater.engine(templater.parse)
     for scheme, url in schemes.items():
-        _check_drive_letter(schemes)
+        _check_drive_letter(scheme)
         url_scheme = urlutil.url(url).scheme
         if url_scheme in hg.peer_schemes:
             hg.peer_schemes[scheme] = ShortRepository(url, scheme, t)
         else:
             hg.repo_schemes[scheme] = ShortRepository(url, scheme, t)
 
     extensions.wrapfunction(urlutil, b'hasdriveletter', hasdriveletter)
 
 
 @command(b'debugexpandscheme', norepo=True)
 def expandscheme(ui, url, **opts):
     """given a repo path, provide the scheme-expanded path"""
     scheme = urlutil.url(url).scheme
     if scheme in hg.peer_schemes:
         cls = hg.peer_schemes[scheme]
     else:
         cls = hg.repo_schemes.get(scheme)
     if cls is not None and isinstance(cls, ShortRepository):
         url = cls.resolve(url)
     ui.write(url + b'\n')