import-checker: allow symbol imports from typing module

As we add typing annotations, we'll want to use a lot of symbols from
the `typing` module. Typing `typing` all the time will be annoying. Let's
allow symbol imports from this module.

While I was here, I changed some comments from "whitelist" to "allow list"
as the former is non-inclusive terminology.

Differential Revision: https://phab.mercurial-scm.org/D12365

contrib/import-checker.py

@@ -19,7 +19,7 b' if True: # disable lexical sorting chec'
 
 import testparseutil
 
-# Whitelist of modules that symbols can be directly imported from.
+# Allow list of modules that symbols can be directly imported from.
 allowsymbolimports = (
     '__future__',
     'breezy',
@@ -46,9 +46,10 b' allowsymbolimports = ('
     'mercurial.thirdparty.attr',
     'mercurial.thirdparty.zope',
     'mercurial.thirdparty.zope.interface',
+    'typing',
 )
 
-# Whitelist of symbols that can be directly imported.
+# Allow list of symbols that can be directly imported.
 directsymbols = ('demandimport',)
 
 # Modules that must be aliased because they are commonly confused with