Switch default python version for PROGRAM/LIBRARY

This also
- prohibits Python macros (PY_SRCS, PY_REGISTER and PY_MAIN) without explicit Python version
- Allows PY_SRCE for PROTO_LIBRARY by ingnoring it in all submodules except PY2_PROTO/PY3_PROTO

Known caveats:
- LIBRARY/PROGRAM still have implicit default (Python3 starting this PR), so Python version mismatches are still possible on pure C/C++ code depending on Python.

- `PY_ANY_MODULE` also switched default, but it's better to require explicit version specification somehow.

- `ALL_PY_SRCS` unlike `PY_SRCS` is not enabled in `PROTO_LIBRARY` and will produce issue of "Unknown python version". `IF (PY2_PROTO OR PY3_PROTO)` is required around such macro to be used. There is only 1 such case in Arcadia and it looks completely weird to me.

- `PYTHON_TYPE_FOR_CYTHON` is now aligned with `PYTHONx_MODULE` module, so all explicit uses are to be removed. Most probably this should be completely removed in favor of `_PYTHON_VER`, but this requires prohibition of PY_ANY_MODULE w/o version and deeper investigation on what `PYTHON_TYPE_FOR_CYTHON` actually is.
commit_hash:e6b809da1352c05e708e070a059306c19d49b53a

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