Stop assuming that /usr/bin/python exists

Fixes 33192

scripts/maint/practracker/includes.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 # Copyright 2018 The Tor Project, Inc.  See LICENSE file for licensing info.
 
 """This script looks through all the directories for files matching *.c or

scripts/maint/practracker/metrics.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 # Implementation of various source code metrics.
 # These are currently ad-hoc string operations and regexps.

scripts/maint/practracker/practracker.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 """
 Best-practices tracker for Tor source code.

scripts/maint/practracker/practracker_tests.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 """Some simple tests for practracker metrics"""