bpo-31650: PEP 552 (Deterministic pycs) implementation

[benjaminp]

Python now supports checking bytecode cache up-to-dateness with a hash of the
source contents rather than volatile source metadata. See the PEP for details.

While a fairly straightforward idea, quite a lot of code had to be modified due
to the pervasiveness of pyc implementation details in the codebase. Changes in
this commit include:

• The core changes to importlib to understand how to read, validate, and
  regenerate hash-based pycs.

• Support for generating hash-based pycs in py_compile and compileall.

• Modifications to our siphash implementation to support passing a custom
  key. We then expose it to importlib through _imp.

• Updates to all places in the interpreter, standard library, and tests that
  manually generate or parse pyc files to grok the new format.

• Support in the interpreter command line code for long options like
  --check-hash-based-pycs.

• Tests and documentation for all of the above.

https://bugs.python.org/issue31650