Use portable python shebangs

* In conda or Gentoo Prefix, we don't want to use the system python and
  instead rely on PATH lookup.

python/tests/basic.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python -u
+#!/usr/bin/env python
 import sys
 import libxml2
 # Memory debug specific

python/tests/exslt.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python -u
+#!/usr/bin/env python
 import sys
 import libxml2
 # Memory debug specific

python/tests/extelem.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python -u
+#!/usr/bin/env python
 import sys
 import string
 import libxml2

python/tests/extfunc.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python -u
+#!/usr/bin/env python
 import sys
 import string
 import libxml2

python/tests/pyxsltproc.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python -u
+#!/usr/bin/env python
 #
 # The exercise of rewriting xsltproc on top of the python
 # bindings, not complete yet and shows up the things missing