Use subprocess instead of os.system.

Move sources to a 'rql' subdirectory, drop the NO_SETUPTOOLS code path
(essentially rewriting all of setup.py).  This avoids a conflict between
our parser.py and the stdlib when running setup.py.  Closes #278637.

Popen's stdout is a byte stream.

Allow rql to work with yapps 2.2.0 which is properly released on pypi, and remove the custom dependency link for it.

Closes #124962
Closes #245540

We need this for set comparison implemented in 2a175b352c78