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.

Python/pythonrun.c

@@ -1053,7 +1053,8 @@ run_pyc_file(FILE *fp, const char *filename, PyObject *globals,
                        "Bad magic number in .pyc file");
         return NULL;
     }
-    /* Skip mtime and size */
+    /* Skip the rest of the header. */
+    (void) PyMarshal_ReadLongFromFile(fp);
     (void) PyMarshal_ReadLongFromFile(fp);
     (void) PyMarshal_ReadLongFromFile(fp);
     if (PyErr_Occurred())