Fix Python 3.9 compatibility for FrozenDict due to PEP 584 implementation.

[tirkarthi]

Python 3.9 implements PEP 584 that supports union using | and |=. It also implements __or__, __ior__ and __ror__. In these methods __ior__ mutates the dictionary which shouldn't be done for FrozenDict. Python 3.8 and below raise TypeError since | and |= are not implemented but |= goes through for Python 3.9 hence add it to the list of methods that raise TypeError.

Fixes #270

[mahmoud]

Great sleuthing! I saw threads about that bouncing around python-dev, I should have anticipated it'd cause issues here. Glad someone caught it and fixed it! So thank you.

Test looks good, too, this'll go out in the next release sometime in the next couple weeks I imagine. Thanks again!

[tirkarthi]

Thanks @mahmoud