Specifically request Python 3

If Sphinx is Python 3-only, Sphinx's tooling should be Python 3-only
too.

Signed-off-by: Stephen Finucane <stephen@that.guru>

tests/roots/test-apidoc-toc/mypackage/main.py

@@ -1,4 +1,5 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
+
 import os
 
 import mod_resource

utils/bump_version.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import argparse
 import os

utils/checks.py

@@ -60,7 +60,7 @@ def sphinx_has_header(physical_line, filename, lines, line_number):
 
     # line number correction
     offset = 1
-    if lines[0:1] == ['#!/usr/bin/env python\n']:
+    if lines[0:1] == ['#!/usr/bin/env python3\n']:
         lines = lines[1:]
         offset = 2