Raise an exception on unsupported Python versions

[Rogdham]

In some rare cases, the backports.zstd library can be installed on unsupported Python versions despite the requires-python marker).

This PR introduces a crash with an explicit error message in such cases.

Fixes #64 (see that ticket for more details)

[Rogdham]

Confirmed working as expected (i.e. crashing with explicit error message) in the following cases:

• build time
  • python -m pip install
  • uv pip install
  • python -m build --sdist
  • python -m build --wheel
• install time (from sdist)
  • python -m pip install
  • uv pip install
• run time
  • import backports.zstd
  • from backports import zstd
  • from backports.zstd import compress