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.

1 files changed, 1 insertions, 1 deletions

diff --git a/Lib/pkgutil.py b/Lib/pkgutil.py
index 9180eae..8474a77 100644
--- a/Lib/pkgutil.py
+++ b/Lib/pkgutil.py
@@ -46,7 +46,7 @@ def read_code(stream):
     if magic != importlib.util.MAGIC_NUMBER:
         return None
 
-    stream.read(8) # Skip timestamp and size
+    stream.read(12) # Skip rest of the header
     return marshal.load(stream)