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.

Misc/python.man

@@ -72,6 +72,11 @@ python \- an interpreted, interactive, object-oriented programming language
 ]
 .B \-?
 ]
+.br
+       [
+.B \--check-hash-based-pycs
+\'default\'|\'always\'|\'never\'
+]
 .br
        [
 .B \-c
@@ -123,6 +128,9 @@ Specify the command to execute (see next section).
 This terminates the option list (following options are passed as
 arguments to the command).
 .TP
+.BI "\-\-check-hash-based-pycs " mode
+Configure how Python evaluates the up-to-dateness of hash-based .pyc files.
+.TP
 .B \-d
 Turn on parser debugging output (for expert only, depending on
 compilation options).