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

@@ -135,7 +135,7 b' schemes = {'
 }
 
 
-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
@@ -152,7 +152,7 b' 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)