Add pywin32 as optional dependancy

[nWestie]

Fix for #735

add pywin32 dependency when installing with the [win] option.
Alternative, it could be added as a platform specific dependency for windows(added, but commented out)

I'm not sure which is the most ideal behavior, thoughts?

[misl6]

For now, in order to target issue #735 adding it as an optional dependency is fine (so please remove the additional comment).

In near future (and if you want to, feel free to contribute), I think is better to install platform-specific dependencies by default, in order to be more developer friendly. (Unless the footprint of the platform-specific dependency is huge)

Also, we should consider switching to https://setuptools.pypa.io/en/latest/userguide/declarative_config.html

[nWestie]

Great.
I'd agree that it probably makes sense to add platform dependencies automatically, my only question is how that looks when developing for android/iOS; I assume those would stay as optional dependencies, and the dependencies for mac/win/linux would be installed anyways?

As far as updating, would it make more switch to the pyproject.toml standard instead?
https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html
My impression is that it is the most up to date format, so it could make sense to switch to that if there is no specific reason not to