Port to Python 3.9

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On Python 3.9, define _Py_DEC_REFTOTAL which has been removed by:
python/cpython@49932fe

Replace also PyEval_CallObjectWithKeywords() with PyObject_Call(),
since PyEval_CallObjectWithKeywords() has been deprecated in
Python 3.9 and PyObject_Call() has the same behavior. The only
difference is that PyEval_CallObjectWithKeywords() can be called with
args=NULL, but g_initialstub() ensures that args is not NULL.

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I tested manually that tests pass on Python 3.9 on Fedora 31:

$ git clean -fdx
$ tox -e py39
(...)
Ran 63 tests in 0.464s
(...)
  py39: commands succeeded
  congratulations :)

[snaury]

Thank you!