Ensure files are closed properly, to avoid Python3 warning
Ensure the test file is closed after use to prevent Python3 warnings
Unit test fixes for Python3 support
Monkey-patch httpretty for Python3 support

This fix is in the httpretty codebase, but not yet released
requests-oauthlib 0.3.2 introduced a Unicode bug.

Hold back to 0.3.1 until this is resolved
Travis now tests all supported Python versions