extras.py is still present on version 2.0.0

[azhpushkin]

Steps to reproduce: install latest version from pypi

There is no extras.py in source code anymore but it is still present after installation. This creates certain confusion as two versions of KeysSubset are present (in keys.py and extras).
Moreover, documentation is still pointing to the extras, which is even more confusing

[azhpushkin]

@Deepwalker This is not urgent at all, but I guess you are the only one who can update the build 🤞

[Arfey]

thx for your contribution. MR u welcome.

[sonerayberk]

@Arfey I'm afraid, but isn't the build can be built and deployed to PyPI by the author of the library?

[Arfey]

Sure, but will be great if u fix documentation :)

[Deepwalker]

Have no idea how it happened. Issued 2.0.1.