Log message:
extend PYTHON_VERSIONS_ for Python 3.7

Log message:
Add python-3.6 to incompatible versions.

Log message:
Remove python33: adapt all packages that refer to it.

Log message:
Extend PYTHON_VERSIONS_INCOMPATIBLE to 35

Log message:
Mark packages that are not ready for python-3.3 also not ready for 3.4,
until proven otherwise.

Log message:
Mark packages as not ready for python-3.x where applicable;
either because they themselves are not ready or because a
dependency isn't. This is annotated by
PYTHON_VERSIONS_INCOMPATIBLE=  33 # not yet ported as of x.y.z
or
PYTHON_VERSIONS_INCOMPATIBLE=  33 # py-foo, py-bar
respectively, please use the same style for other packages,
and check during updates.

Use versioned_dependencies.mk where applicable.
Use REPLACE_PYTHON instead of handcoded alternatives, where applicable.
Reorder Makefile sections into standard order, where applicable.

Remove PYTHON_VERSIONS_INCLUDE_3X lines since that will be default
with the next commit.

Whitespace cleanups and other nits corrected, where necessary.

Log message:
Drop superfluous PKG_DESTDIR_SUPPORT, "user-destdir" is default these days.

Log message:
All supported python versions in pkgsrc support eggs, so remove
${PLIST.eggfile} from PLISTs and support code from lang/python.