oauthlib no longer supports python2.7

Hi,

We are using the 3.1 tag and have builds failing due to the latest version of [oauthlib](https://pypi.org/project/requests-oauthlib/1.4.0/) which no longer supports python2.7.

We think the best way to solve this is to pin the version of oauthlib [here](https://github.com/bear/python-twitter/blob/1a148ead5029d06bec58c1cbc879764aa4b2bc74/requirements.txt#L2) to 1.3

Cheers