Support Python >=3.10,<4

[tachylatus]

The package builds and runs with Python 3.10 (also in development mode).
Originally posted by @gboehl in sdv-dev/Copulas#195 (comment)

Python 3.10 was first released on 2021-10-04, and Python 3.11 is right around the corner.

You should either change the python_requires version constraint to >=3.6,<4 or remove it entirely.
If it would ease the maintenance burden to drop support for Python 3.6, consider doing so as well, as it has not been supported for over a year now.

[npatki]

Hi @tachylatus, thanks for filing this. Seems like this could ease the burden for both users and the maintainers. I'm moving it into our main SDV library, where we track issues that apply to all libraries in our ecosystem (including Copulas)

Note: For the support of Python 3.10 explicitly, there is an existing issue -- #939.

[amontanez24]

Resolved by #1179