xbps-src: introduce python_version

Manual.md

@@ -1107,6 +1107,11 @@ By default it's set to `2.7` for `python 2.x` packages.
 
 > NOTE: you need to define it *only* for non-Python modules.
 
+- `python_version`: this variable expects the supported Python major version.
+By default it's set to `2`. This variable is needed for multi-language
+applications (e.g., the application is written in C while the command is
+written in Python) or just single Python file ones that live in `/usr/bin`.
+
 Also, a set of useful variables are defined to use in the templates:
 
 | Variable    | Value                            |

common/environment/setup/python.sh

@@ -4,6 +4,9 @@
 __python2="/usr/bin/python2"
 __python3="/usr/bin/python3"
 
+# set version 2 as the default Python
+python_version="2"
+
 if [ -x ${__python2} ]; then
   py2_ver="$(${__python2} -c 'import sys; print(sys.version[:3])')"
   py2_lib="$(${__python2} -c 'from distutils.sysconfig import get_python_lib; print(get_python_lib(0, 1))')"

common/environment/setup/sourcepkg.sh

@@ -5,7 +5,7 @@
 unset -v pkgname version revision short_desc homepage license maintainer
 unset -v only_for_archs distfiles checksum build_style nocross broken
 unset -v configure_script configure_args wrksrc build_wrksrc create_wrksrc
-unset -v make_cmd make_build_args make_install_args make_build_target make_install_target stackage
+unset -v make_cmd make_build_args make_install_args make_build_target make_install_target python_version stackage
 unset -v patch_args disable_parallel_build keep_libtool_archives
 unset -v reverts subpackages makedepends hostmakedepends depends restricted
 unset -v nopie build_options build_options_default bootstrap repository reverts

common/hooks/pre-pkg/03-rewrite-python-shebang.sh

@@ -14,6 +14,10 @@ hook() {
 		pyver="$pycompile_version"
 	fi
 
+	if [ "$python_version" = "3" ]; then
+		pyver="$python_version"
+	fi
+
 	shebang="#!/usr/bin/python${pyver%.*}"
 	find ${PKGDESTDIR} -type f -print0 | \
 		xargs -0 grep -H -b -m 1 "^#!.*\([[:space:]]\|/\)python\([[:space:]]*\|$\)" -- | while IFS=: read -r f off _; do