Vim fails to build with Python3.9

[Zdenek Dohnal]

Hi,

our Fedora python team is doing checks of packages which requires python about their issues with new python version - 3.9.

They found out Vim fails to build from source with python3.9 due following error:

if_python3.c: In function 'py3__Py_DECREF':
if_python3.c:606:5: error: '_Py_DEC_REFTOTAL' undeclared (first use in this function); did you mean '_Py_DECREF'?
  606 |     _Py_DEC_REFTOTAL;
      |     ^~~~~~~~~~~~~~~~
      |     _Py_DECREF
if_python3.c:606:5: note: each undeclared identifier is reported only once for each function it appears in

_Py_DEC_REFTOTAL macro is no longer available in python3.9 - Python project uses variable decrementing directly now '_Py_RefTotal--;'

Actually, I'm not quite sure Vim defines py3__Py_DECREF function in its python module, when it does the same thing as _Py_DECREF defined in python library.

[Bram Moolenaar]

Looks like it was only defined for debugging. I think we can just drop
that line.

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[Zdenek Dohnal]

Thank you for the fix!