[BUG] gitversion: Clean ubutntu install does not have python but has python3

[bozzzzo]

Describe the bug
cmake fails with failing to execute python

To Reproduce

1. cmake ..

Defining simavr location as /home/foo/Projects/MK404/3rdParty/simavr/simavr/obj-x86_64-linux-gnu/libsimavr.a
/usr/bin/env: ‘python’: No such file or directory
CMake Error at 3rdParty/gitversion/cmake.cmake:18 (MESSAGE):
  Error running messmer/git-version tool.  Return code is: 127
Call Stack (most recent call first):
  3rdParty/gitversion/cmake.cmake:39 (_CREATE_GIT_VERSION_FILE)
  CMakeLists.txt:217 (TARGET_GIT_VERSION_INIT)


-- Configuring incomplete, errors occurred!

Expected behavior
A working build script

Desktop (please complete the following information):
Description: Ubuntu 20.04.1 LTS
Release: 20.04
Codename: focal

I made a PR smessmer/gitversion#9

[vintagepc]

Nice. We'd probably have hit this once the ubuntu-latest buildrunner becomes 20.04LTS (it's currently still 18).
We'll pull that in once its updated upstream.

[bozzzzo]

I find it good practice to fork all submodule projects. This allows fixing small stuff like this
before the upstream reacts.

[wavexx]

Hit this as well on debian unstable... uurgh.

[vintagepc]

yeah, looking deeper upstream seems to be stale. I'll fork/merge/whatever at some point.