package/python3: add upstream security fix for CVE-2021-3177

package/python3: add upstream security fix for CVE-2021-3177

Fixes the following security issue:

- CVE-2021-3177: Python 3.x through 3.9.1 has a buffer overflow in
  PyCArg_repr in _ctypes/callproc.c, which may lead to remote code execution
  in certain Python applications that accept floating-point numbers as
  untrusted input, as demonstrated by a 1e300 argument to
  c_double.from_param.  This occurs because sprintf is used unsafely.

For details, see the advisory:
https://python-security.readthedocs.io/vuln/ctypes-buffer-overflow-pycarg_repr.html

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>