I revised the patch with respect to Alexander's comments. In summary,
here is what I changed from the previous patch:

  - Removed the unnecessary "fixes" to Objects/structseq.c
    and Modules/timemodule.c
  - Updated Objects/typeobject.c to forbid null-bytes in __doc__,
    since they cannot be handled in the `tp_doc` member.
  - Removed an erroneous pointer dereference in Modules/zipimport.c
  - Changed `len+1` to `len` in the memcpy() call in the
    Modules/parsermodule.c fixes