Frozendict

Now frozendict C extension compiles also for Python 3.10

**Full Changelog**: https://github.com/Marco-Sulla/python-frozendict/compare/v2.0.4...v2.0.7

Now 'cls' can be a key in the pure py version

Inhibited the update operator `__ior__()` (`|=`) introduced in Python 3.9

PS: no pure Linux C builds, since they can't be published on Pypi. For now I'm focusing on the pure py implementation

Various improvements to the C implementation.

* C extension now supports also Python 3.8 on Linux.
* Added wheel for the pure Python version.

Added a C extension. Speedup everywhere, sometimes faster than dict! C extension for now only for Linux and Python 3.9.