build gdbgui as python executable (pex)

[cs01]

• I have added an entry to CHANGELOG.md, or an entry is not needed for this change

Summary of changes

• Build gdbgui as a pex executable.
  • These are executable Python environments that are self-contained with the exception of requiring a specific Python version installed in the environment running the executable. The pex executables should have better compatibility than PyInstaller executables, which sometimes have missing shared libraries depending on the operating system.
• Included in github action, as well as a nox session

Test plan

Tested by running

> nox -s build_pex
~/git/gdbgui/build/pex csmith@gram (cs01/add-pex*)
> md5sum gdbgui.pex
bb65703f2aa5dfef60a2b475a8341d69  gdbgui.pex
~/git/gdbgui/build/pex csmith@gram (cs01/add-pex*)
> cat gdbgui.pex.md5 
bb65703f2aa5dfef60a2b475a8341d69
> ./build/pex/gdbgui.pex