[F823] Warn when variable name is referenced before assignment

[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