message: Add Support for building the Python API against Python 3.9
diff: <PythonOverride Condition="'$(Platform)'=='x64'">$(RENDERDOC_PYTHON_PREFIX64)</PythonOverride>
  </PropertyGroup>
  +<PropertyGroup><PythonMajorMinorTest>39</PythonMajorMinorTest></PropertyGroup>
  +<PropertyGroup Condition="'$(CustomPythonUsed)'=='0' AND Exists('$(PythonOverride)\include\Python.h') AND Exists('$(PythonOverride)\python$(PythonMajorMinorTest).zip') AND (Exists('$(PythonOverride)\python$(PythonMajorMinorTest).lib') OR Exists('$(PythonOverride)\libs\python$(PythonMajorMinorTest).lib'))"><CustomPythonUsed>$(PythonMajorMinorTest)</CustomPythonUsed></PropertyGroup>
  +
  <PropertyGroup><PythonMajorMinorTest>38</PythonMajorMinorTest></PropertyGroup>
  <PropertyGroup Condition="'$(CustomPythonUsed)'=='0' AND Exists('$(PythonOverride)\include\Python.h') AND Exists('$(PythonOverride)\python$(PythonMajorMinorTest).zip') AND (Exists('$(PythonOverride)\python$(PythonMajorMinorTest).lib') OR Exists('$(PythonOverride)\libs\python$(PythonMajorMinorTest).lib'))"><CustomPythonUsed>$(PythonMajorMinorTest)</CustomPythonUsed></PropertyGroup>
  