Use python2 to build debugpy-python2

[sbivol]

Fixes #936

There must be a python2 interpreter available, which is the case on Linux when Python 2 is installed.

On Windows, one needs to link (or move) python.exe to python2.exe, otherwise it's not possible to distinguish between Python 2 and Python 3 when both are on %PATH%:

cd C:\Python27\
mklink /H python2.exe python.exe

Now the gadget can be installed successfully.

[puremourning]

This change is 

[puremourning]

I'm not sure about this. I get that it's broken and that's annoying, but at the same time, having to create a python2 symlink to make it work is just as broken.

My temptation at this point actually is to just remove this python2 support from the main vimspector and write a wiki page that says how to set it up manually. Python2 has been deprecated for so long now, I don't really feel like supporting it any longer directly.

[sbivol]

Thanks for looking into this.

Linking is only required on Windows, since all interpreters get the python.exe name there, irrespective of version.
On Linux there's no such issue, and I expect the same to be true on MacOS.

Vimspector support for Python 2 is valuable to me since it helps with porting abandoned software.

Should you decide to keep this functionality, I volunteer to test debugpy-python2 and fix any issues that may arise with it in the future.