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.

Lib/test/test_importlib/test_abc.py

@@ -673,6 +673,7 @@ class SourceLoader(SourceOnlyLoader):
         if magic is None:
             magic = self.util.MAGIC_NUMBER
         data = bytearray(magic)
+        data.extend(self.init._w_long(0))
         data.extend(self.init._w_long(self.source_mtime))
         data.extend(self.init._w_long(self.source_size))
         code_object = compile(self.source, self.path, 'exec',
@@ -836,6 +837,7 @@ class SourceLoaderBytecodeTests(SourceLoaderTestHarness):
         if bytecode_written:
             self.assertIn(self.cached, self.loader.written)
             data = bytearray(self.util.MAGIC_NUMBER)
+            data.extend(self.init._w_long(0))
             data.extend(self.init._w_long(self.loader.source_mtime))
             data.extend(self.init._w_long(self.loader.source_size))
             data.extend(marshal.dumps(code_object))