Avoid Python3 "list.remove(x): x not in list" error

Every time TextMate2 updates (and refreshes this file), I get the following error running in-app with Python3 (Python works fine).
```
Error in sitecustomize; set PYTHONVERBOSE for traceback: ValueError: list.remove(x): x not in list
```
It looks like my sitecustomize is being run twice with Python3 (lines 20-25), which is provoking the error; the `os.environ['TM_BUNDLE_SUPPORT']` is removed the first time, so it gives an error trying to remove it the second.

This problem has also been looked at [here](https://github.com/mxcl/homebrew/pull/16848) and [here](https://github.com/textmate/textmate/issues/812).