patch 8.0.0251: not easy to select Python 2 or 3

Problem:    It is not so easy to write a script that works with both Python 2
            and Python 3, even when the Python code works with both.
Solution:   Add 'pyxversion', :pyx, etc. (Marc Weber, Ken Takata)
diff --git a/src/Makefile b/src/Makefile
index f993c85..c675c06 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2151,6 +2151,8 @@
 	test_popup \
 	test_profile \
 	test_put \
+	test_pyx2 \
+	test_pyx3 \
 	test_quickfix \
 	test_regexp_latin \
 	test_regexp_utf8 \
@@ -2754,6 +2756,7 @@
 				 ../../testdir/*.vim \
 				 ../../testdir/*.py \
 				 ../../testdir/python* \
+				 ../../testdir/pyxfile \
 				 ../../testdir/sautest \
 				 ../../testdir/samples \
 				 ../../testdir/test83-tags? \