An implementation note mostly to myself: In 3.8 and before, 'python relative/path/to/file.py' results in the __file__ attribute of the main module being the relative path, and a possibly unnormalized full path is os.curdir + __file__.  I believe that this is the only situation in which __file__ is not absolute.  After os.chdir(new cwd), relative __file__ becomes useless unless the original curdir is saved or  __file__ is made absolute first.  The startup part of pyshell has this vulnerable line.
    icondir = os.path.join(os.path.dirname(__file__), 'Icons')
But with curdir left alone when starting from a command line, there will be no problem.