Un-hardcode the supported factors again

[asottile]

Resolves #1374

This also fixes some other things which regressed in #983 -- notably this

$ tox -e py21 --notest && .tox/py21/bin/python --version
py21 create: /tmp/tox/.tox/py21
py21 installdeps: pip == 19.1.1
py21 inst: /tmp/tox/.tox/.tmp/package/1/tox-3.13.3.dev7+g84c4c8d.tar.gz
py21 installed: apipkg==1.5,atomicwrites==1.3.0,attrs==19.1.0,coverage==4.5.3,execnet==1.6.1,filelock==3.0.12,flaky==3.6.0,freezegun==0.3.12,importlib-metadata==0.18,more-itertools==7.2.0,packaging==19.0,pathlib2==2.3.4,pluggy==0.12.0,psutil==5.6.3,py==1.8.0,pyparsing==2.4.1.1,pytest==5.0.1,pytest-cov==2.7.1,pytest-forked==1.0.2,pytest-mock==1.10.4,pytest-randomly==1.2.3,pytest-xdist==1.29.0,python-dateutil==2.8.0,six==1.12.0,toml==0.10.0,tox==3.13.3.dev7+g84c4c8d,virtualenv==16.7.1,wcwidth==0.1.7,zipp==0.5.2
_______________________________________________________________________ summary ________________________________________________________________________
  py21: skipped tests
  congratulations :)
Python 3.6.8

restored behaviour:

$ .tox/py36/bin/tox -e py21 --notest
py21 create: /tmp/tox/.tox/py21
SKIPPED: InterpreterNotFound: python2.1
_______________________________________________________________________ summary ________________________________________________________________________
SKIPPED:  py21: InterpreterNotFound: python2.1
  congratulations :)

[todo]

(stephenfin): Raise an exception here in tox 4.0

tox/src/tox/config/__init__.py

Lines 577 to 582 in 84c4c8d

	# TODO(stephenfin): Raise an exception here in tox 4.0
	warnings.warn(
	"conflicting basepython version (set {}, should be {}) for env '{}';"
	"resolve conflict or set ignore_basepython_conflict".format(
	proposed_version, implied_version, testenv_config.envname
	)

---

This comment was generated by todo based on a TODO comment in 84c4c8d in #1377. cc @asottile.

[gaborbernat]

Looks good 👍 if the ci passes 👍

[asottile]

the py34 failures appear to be due to #1375

[gaborbernat]

Can you fix the merge conflict? How we fix the py34 thing until upstream fixes it?

[asottile]

I think we can pyparsing!=2.4.1,!=2.4.1.1 to work around the bug -- let me make a PR and see if that's doable