After trying to make a patch I found what current test suite itself has calls like (str, shell=False), (bytes, shell=True) and (['shell command'], shell=True).

We can:
1. Implement Eric's suggestion with fixing/removing broken tests.
2. Add Pope.shell and Popen.exec as Nick propose.

I don't like former because it changes already existing contracts fixed and published by explicit unittests without strong reason. Also Nick pointed reasons why user may need existing functionality. Adding new 'safe' shell and exec classmethods as prime entrypoints for Popen make sense.