Use any python version for python bindings

In order to work on Ubuntu 20.04 which does not have a "python" lib,
only "python3" and "python2".

See https://www.fosslinux.com/26413/ubuntu-20-04-lts-focal-fossa-set-to-remove-python-2.htm.

Also needed was to replace pyversion which found only Python 2
versions. Possibly this can done in a better way by someone who
understands Debian packaging in general and Lintian in particular.

See https://lintian.debian.org/tags/debian-pyversions-is-obsolete.html.

debian/libgv-python.install

-usr/lib/graphviz/python
+usr/lib/graphviz/python*
 #usr/lib/graphviz/python24
 #usr/lib/graphviz/python25
 #usr/lib/graphviz/python26

debian/rules

@@ -33,7 +33,7 @@ LUA_PACKAGE       = $(CURDIR)/debian/libgv-lua
 
 DEV_PACKAGE       = $(CURDIR)/debian/libgraphviz-dev
 
-PYTHON_VERSIONS   = $(shell pyversions -s)
+PYTHON_VERSIONS   = $(shell python2 --version |& sed 's/.* \(.*\)\..*/\1/'; python3 --version |& sed 's/.* \(.*\)\..*/\1/')
 PYTHON_PACKAGE    = $(CURDIR)/debian/libgv-python
 
 RUBY_VERSION      = 1.8