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.

Include/internal/hash.h

@@ -0,0 +1,6 @@
+#ifndef Py_INTERNAL_HASH_H
+#define Py_INTERNAL_HASH_H
+
+uint64_t _Py_KeyedHash(uint64_t, const char *, Py_ssize_t);
+
+#endif

Include/internal/import.h

@@ -0,0 +1,6 @@
+#ifndef Py_INTERNAL_IMPORT_H
+#define Py_INTERNAL_IMPORT_H
+
+extern const char *_Py_CheckHashBasedPycsMode;
+
+#endif