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_py_compile.py

@@ -122,6 +122,24 @@ class PyCompileTests(unittest.TestCase):
         # Specifying optimized bytecode should lead to a path reflecting that.
         self.assertIn('opt-2', py_compile.compile(self.source_path, optimize=2))
 
+    def test_invalidation_mode(self):
+        py_compile.compile(
+            self.source_path,
+            invalidation_mode=py_compile.PycInvalidationMode.CHECKED_HASH,
+        )
+        with open(self.cache_path, 'rb') as fp:
+            flags = importlib._bootstrap_external._classify_pyc(
+                fp.read(), 'test', {})
+        self.assertEqual(flags, 0b11)
+        py_compile.compile(
+            self.source_path,
+            invalidation_mode=py_compile.PycInvalidationMode.UNCHECKED_HASH,
+        )
+        with open(self.cache_path, 'rb') as fp:
+            flags = importlib._bootstrap_external._classify_pyc(
+                fp.read(), 'test', {})
+        self.assertEqual(flags, 0b1)
+
 
 if __name__ == "__main__":
     unittest.main()