Prefer python 3, and mention intent to drop python 2 support

Prefer python 3, and mention intent to drop python 2 support

For now, set the minimum supported 3 version at 3.7 since I just
tested that on a debian/buster system, and update the installation
instructions after seeing what was required to run the tests from
scratch there.  Continue to search for 3.6 and 2.6 for now, since no
one's complained, and we can always lower the requirement later if it
seems appropriate.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>