Xpra: Ticket #2033: setuptools win32 breakage

It's not a bug, it's a feature: Drop support for self upgrade/installation.

And here what it looks like:

$ PYTHON=python2 sh win32/MINGW_BUILD.sh
Python 2.7.15
Python2 version
Xpra 2.5-r20978 (64-bit)
* Cleaning ./dist output directory
* Building Python 2 Cython modules
* Generating installation directory
ERROR: build failed, see win32/cx_freeze-install.log:
    self.run_command(cmd)
  File "C:/msys64/mingw64/lib/python2.7/distutils/dist.py", line 972, in run_command
    cmd_obj.run()
  File "C:/msys64/mingw64/lib/python2.7/site-packages/cx_Freeze/dist.py", line 305, in run
    self.run_command('build_exe')
  File "C:/msys64/mingw64/lib/python2.7/distutils/cmd.py", line 326, in run_command
    self.distribution.run_command(command)
  File "C:/msys64/mingw64/lib/python2.7/distutils/dist.py", line 972, in run_command
    cmd_obj.run()
  File "C:/msys64/mingw64/lib/python2.7/site-packages/cx_Freeze/dist.py", line 219, in run
    freezer.Freeze()
  File "C:/msys64/mingw64/lib/python2.7/site-packages/cx_Freeze/freezer.py", line 621, in Freeze
    self.finder = self._GetModuleFinder()
  File "C:/msys64/mingw64/lib/python2.7/site-packages/cx_Freeze/freezer.py", line 340, in _GetModuleFinder
    finder.IncludeModule(name)
  File "C:/msys64/mingw64/lib/python2.7/site-packages/cx_Freeze/finder.py", line 651, in IncludeModule
    namespace = namespace)
  File "C:/msys64/mingw64/lib/python2.7/site-packages/cx_Freeze/finder.py", line 351, in _ImportModule
    raise ImportError("No module named %r" % name)
ImportError: No module named 'pkg_resources._vendor.packaging'