Deprecate extra values for srcs_version.

[brandjon]

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

[brandjon]

Accompanying the fix for this and #1393, we'll also make the error message less confusing for when you have the wrong major python version.

[brandjon]

One question raised by these names is: What happens when there is a Python 4? At present it looks like while there are some references in PEPs and mailing lists to "Python 4", that's used as a stand-in to mean "a breaking change release". Alternative interpretations are that Python 4 will just be Python 3.10. Either way, until more concrete plans emerge from the Python developers, no sense speculating in our plans here. Python 2 compatibility features may very well be deprecated by the time it becomes a concern.

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