[F812] Warn when list comprehension redefines a name

[ansley]

Stack from ghstack (oldest at bottom):

• [F823] Warn when variable name is referenced before assignment #277
• -> [F812] Warn when list comprehension redefines a name #276
• [F811] Warn on and delete double import #275
• [F403, F405, F821] Warn on undefined names #274
• [F403] Warn on star import #273
• [F402] Warn on loop variable shadowing module name #272
• Test rules in pyflakes directory #271

This lint rule is probably in Flake8 to prevent elements from leaking
out of the comprehension scope, but the behavior was fixed in Python3.
I worry such a strict lint rule will cause more problems than it solves,
considering we use Python 3.10 at Meta. Perhaps we should consider
disabling this rule by default.

[amyreese]

Can you add some test cases covering mutually exclusive scopes, where I would expect this to be allowed?

Something like:

def foo():
    i = 1
    print(i)

def bar():
    values = [i * 2 for i in range(5)]