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_imp.py

@@ -4,11 +4,13 @@ import os
 import os.path
 import sys
 from test import support
+from test.support import script_helper
 import unittest
 import warnings
 with warnings.catch_warnings():
     warnings.simplefilter('ignore', DeprecationWarning)
     import imp
+import _imp
 
 
 def requires_load_dynamic(meth):
@@ -329,6 +331,25 @@ class ImportTests(unittest.TestCase):
         with self.assertRaises(TypeError):
             create_dynamic(BadSpec())
 
+    def test_source_hash(self):
+        self.assertEqual(_imp.source_hash(42, b'hi'), b'\xc6\xe7Z\r\x03:}\xab')
+        self.assertEqual(_imp.source_hash(43, b'hi'), b'\x85\x9765\xf8\x9a\x8b9')
+
+    def test_pyc_invalidation_mode_from_cmdline(self):
+        cases = [
+            ([], "default"),
+            (["--check-hash-based-pycs", "default"], "default"),
+            (["--check-hash-based-pycs", "always"], "always"),
+            (["--check-hash-based-pycs", "never"], "never"),
+        ]
+        for interp_args, expected in cases:
+            args = interp_args + [
+                "-c",
+                "import _imp; print(_imp.check_hash_based_pycs)",
+            ]
+            res = script_helper.assert_python_ok(*args)
+            self.assertEqual(res.out.strip().decode('utf-8'), expected)
+
 
 class ReloadTests(unittest.TestCase):