pkgsrc python default version -> 3.7

[coypu%sdf.org@localhost]

Hi folks,

The default Python version in pkgsrc is now 3.7, in preparation for
the coming end of life date of Python 2.7 (the previous default) at
the end of this year.

This means any package that can be built with Python 3.7 will be built
with it, rather than Python 2.7.
Packages with no Python 3.x support will continue to be built with
Python 2.7.

Problems are not likely to occur as many developers have been using this
default for a while.
Let me know if there are any problems still.

To undo this change, if you have a need for it, add this line to your
/etc/mk.conf:
PYTHON_VERSION_DEFAULT=27

For binary packages named pyXX-packagename, a python 2.7 will still be
built (py27-packagename), as well as all the other python versions
available.