chore(types): update mypy version

[daniel-sanche]

fix a couple typing issues that were causing recent versions of mypy to fail:

• exceptions.py: error_info variable was reused for a list, and for an optional dict type. Broke the usages into separate variables
• grpc_helpers_async: added Generic to _WrappedCall
• retry_unary: removed _P arguments from target type annotation. We expect target to be a functools.partial, and have no arguments

Fixes #682

[parthea]

Thanks @daniel-sanche !