contextlib: get rid of a TODO (#4610)

This dates back three years to https://github.com/python/typeshed/pull/1249

It seems pretty unused in practice, so I think this is fine:
https://grep.app/search?q=from%20contextlib%20import%20ContextManager&case=true
https://grep.app/search?q=contextlib.ContextManager&case=true

Co-authored-by: hauntsaninja <>

tests/stubtest_whitelists/py3_common.txt

@@ -102,7 +102,6 @@ configparser.SectionProxy.__getattr__
 configparser.SectionProxy.getboolean
 configparser.SectionProxy.getfloat
 configparser.SectionProxy.getint
-contextlib.ContextManager
 csv.Dialect.delimiter
 csv.Dialect.doublequote
 csv.Dialect.lineterminator