test: add context manager tests for __enter__ behavior

[lidorcg]

fix: update 'with' macro to bind return value of __enter__ correctly

https://github.com/clj-python/libpython-clj/issues/273

[jjtolton]

Good catch. Technically this is a backwards breaking change but I sincerely doubt anyone is explicitly expecting the bound/return value to always be nil and taking conditional logic from that.

Traveling right now for conference stuff but I will pull it and check it when I can.

Thanks for the contribution!

[jjtolton]

@lidorcg overall looks good to me. Tested locally and working great. Would prefer you revert the unrelated formatting changes and do that in separate PR. Excellent contribution, thank you!