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/importlib/util.py

@@ -5,18 +5,25 @@ from ._bootstrap import _resolve_name
 from ._bootstrap import spec_from_loader
 from ._bootstrap import _find_spec
 from ._bootstrap_external import MAGIC_NUMBER
+from ._bootstrap_external import _RAW_MAGIC_NUMBER
 from ._bootstrap_external import cache_from_source
 from ._bootstrap_external import decode_source
 from ._bootstrap_external import source_from_cache
 from ._bootstrap_external import spec_from_file_location
 
 from contextlib import contextmanager
+import _imp
 import functools
 import sys
 import types
 import warnings
 
 
+def source_hash(source_bytes):
+    "Return the hash of *source_bytes* as used in hash-based pyc files."
+    return _imp.source_hash(_RAW_MAGIC_NUMBER, source_bytes)
+
+
 def resolve_name(name, package):
     """Resolve a relative module name to an absolute one."""
     if not name.startswith('.'):