xbps-src: add support for python3 pkgs

- python_module build style now builds modules for python2/3 by default
- new python2_module and python3_module build styles for building
  python2-only and python3-only packages respectively
- no more python_versions
- no need to define pycompile_version for Python modules anymore
  (still needed for non-Python modules though)
- Python version and paths are now guessed automatically and a set of
  useful variables can now be used in templates
- #!/usr/bin/python2 and #!/usr/bin/python3 are now the default shebangs
- /usr/bin/foo2 and /usr/bin/foo3 are now the default names for bin
  scripts (for use with alternatives)

common/hooks/post-install/04-create-xbps-metadata-scripts.sh

@@ -231,6 +231,10 @@ _EOF
 	#
 	# Handle python bytecode archives with pycompile trigger.
 	#
+	if [ -d ${PKGDESTDIR}/usr/lib/python* ]; then
+		pycompile_version="$(find ${PKGDESTDIR}/usr/lib/python* -type d | grep -o '[[:digit:]]\.[[:digit:]]$')"
+	fi
+
 	if [ -n "${pycompile_dirs}" -o -n "${pycompile_module}" ]; then
 		echo "export pycompile_version=\"${pycompile_version:=2.7}\"" >>$tmpf
 		if [ -n "${pycompile_dirs}" ]; then