Current allowed srcs_version values are:

• PY2ONLY: errors out if built in PY3 mode
• PY2: originally intended to mean "do 2to3 conversion", but it errors out (badly) if built in PY3 mode
• PY2AND3: works for both PY2 and PY3 (no 2to3 conversion)
• PY3ONLY: errors out if built in PY2 mode
• PY3: same as PY3ONLY

We should reduce this to something like just:

• PY2 (behaving like PY2ONLY does today)
• PY2AND3
• PY3