tools: Fix python -> python3 for bots scripts.

This also turns off the custom lint check that was
preventing an explicit version for scripts in "tools".

Fixes #187

tools/custom_check.py

@@ -165,8 +165,7 @@ def build_custom_checkers(by_lang):
         # This rule might give false positives in virtualenv setup files which should be excluded,
         # and comments which should be rewritten to avoid use of "python2", "python3", etc.
         {'pattern': 'python[23]',
-         'exclude': set(['tools/provision']),
-         'include_only': set(['zulip/', 'tools/', 'zulip_botserver/']),
+         'include_only': set(['zulip/', 'zulip_botserver/']),
          'description': 'Explicit python invocations should not include a version'},
         {'pattern': '__future__',
          'include_only': set(['zulip_bots/zulip_bots/bots/']),