Test Incorporation

Inspired by the recent incidents of change-in-some-scripts resulting in the breaking of other scripts and back-and-forth reversing the change (see https://github.com/acl-org/acl-anthology/commit/8500a12307de69be9b799308dbdf7e57841d9537#r67683715, https://github.com/acl-org/acl-anthology/pull/1836/commits/50531e243ea5b9842cb359c75a2f8851a1b42002 and discussion in https://github.com/acl-org/acl-anthology/pull/1833#issuecomment-1055813438), it seems like we could really use some tests.

While trying to use `pytest` package for the tests, I encountered a problem. The version of`texsoup` package we currently use is ancient and isn't compatible with `pytest`. https://github.com/acl-org/acl-anthology/blob/56145deed72d4b3a43788e5ea7bfe200b12cd2d8/bin/requirements.txt#L18

Upgradinig the `texsoup` version will solve the problem and I think is the best solution (unless for some reason we have to work with the ancient version). Before I dive into changing the code (https://github.com/acl-org/acl-anthology/blob/master/bin/anthology/texmath.py):

- I'm not terribly familiar with texsoup so upgrading the codebase isn't trivia. Any insights/tips that could speed up the process are greatly appreciated!
- Any other suggestions/solutions?

@mjpost @mbollmann