closes bpo-31650: PEP 552 (Deterministic pycs) implementation (#4575)

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.

Doc/using/cmdline.rst

@@ -210,6 +210,20 @@ Miscellaneous options
    import of source modules.  See also :envvar:`PYTHONDONTWRITEBYTECODE`.
 
 
+.. cmdoption:: --check-hash-based-pycs default|always|never
+
+   Control the validation behavior of hash-based ``.pyc`` files. See
+   :ref:`pyc-invalidation`. When set to ``default``, checked and unchecked
+   hash-based bytecode cache files are validated according to their default
+   semantics. When set to ``always``, all hash-based ``.pyc`` files, whether
+   checked or unchecked, are validated against their corresponding source
+   file. When set to ``never``, hash-based ``.pyc`` files are not validated
+   against their corresponding source files.
+
+   The semantics of timestamp-based ``.pyc`` files are unaffected by this
+   option.
+
+
 .. cmdoption:: -d
 
    Turn on parser debugging output (for expert only, depending on compilation