update lint tooling target versions to py3.6+

[graingert]

Scope and purpose

update lint tooling target versions to py3.6+

Contributor Checklist:

• The associated ticket in Trac is here: https://twistedmatrix.com/trac/ticket/10108
• I ran tox -e lint to format my patch to meet the Twisted Coding Standard
• I have created a newsfragment in src/twisted/newsfragments/ (see: News files)
• I have updated the automated tests.
• I have submitted the associated Trac ticket for review by adding the word review to the keywords field in Trac, and putting a link to this PR in the comment; it shows up in https://twisted.reviews/ now.

[graingert]

@wsanchez previously this PR used com2ann to upgrade type annotations, but mypy treats:

class FilePath(AbstractFilePath):
    path = None  # type: Union[bytes, str]

differently to

class FilePath(AbstractFilePath):
    path: Union[bytes, str] = None

the annotation should be:

class FilePath(AbstractFilePath):
    path: Union[bytes, str]

but there's no FilePath.path which is public