Stop requiring pymysql to be installed by default

By adding pymysql to our requirements, we're including what is
essentially an optional backend and we're being opinionated about
what users should be using.

In reality, we're using sqlalchemy so users should be able to
consume sqlite (default) or things like mysql or pymysql.

Let's keep some level of documentation about pymysql for now
(backwards-compat-ish) and iterate on this further eventually.

Change-Id: I14476b5f1e2498a7451d61e940816baa2b2bc89f

requirements.txt

@@ -8,6 +8,5 @@ decorator>=4.0.0
 cliff
 setuptools>=11.3
 ansible!=2.0.2.0,>=2.0.1.0
-pymysql
 pygments
 debtcollector>=1.2.0