Remove attrs from requirements.txt (#373)

We don't depend on this directly---it is a subdependency that we depend
on via other packages. In normal usage of requirements.txt this line
would not cause issues. However, this line causes issues when used with
a later Twisted version (e.g. 18.7) due to the sed in .travis.yml.
We should probably remove this line unless there is a specific reason to
keep it that I have missed.

requirements.txt

@@ -1,4 +1,3 @@
-attrs >= 19.1.*
 configparser
 future
 futures >= 3.0.*