Makes __{a}exit__ method args nullable.

[peterschutt]

This makes the methods compatible with typeshed's ABCs: https://github.com/python/typeshed/blob/5e9589dd75f8a5f39a38a2ed45f098ff62babc7f/stdlib/contextlib.pyi#L40-L54

Closes #874

[coveralls]

coverage: 92.468%. first build
when pulling 8f1b6c5 on peterschutt:cm-exit-method-typing
into 5fac5ba on agronholm:master.

[agronholm]

Thanks, this is a mistake I make from time to time. I know this is how they should look like though!