Add benchmark for connection fields

[jnak]

Add pytest-benchmark so we can easily track performance changes over time.

PS: It's been already approved by @Nabellaleen in my own repo jnak#1

[katy-sadowski]

hello

[jnak]

@Nabellaleen For some reasons that I don't fully understand, our tests for Python 3.4 suddenly stopped working on master. It can no longer install PyYaml, a dependency of pre-commit, one our dev dependencies. I investigated a bit and it looks like it's due to an underlying issue in PyYaml. Given Python 3.4 has reached end of life in March 2019 and it's fairly easy to upgrade from Python 3.4, I don't think we should spend time trying to make the tests pass. I disabled the tests for Python 3.4. Nonetheless, I still expect graphene-sqlalchemy to work with Python 3.4 for the foreseeable future given that we don't use anything specific to Python 3.4.

Alas I also had to upgrade coveralls because of another new sudden issue.