Remove redundant ply entry from requirements.txt (#533)

ply is a transitive dependency of stone and is not imported directly
anywhere in this repo, so listing it explicitly in the dev
requirements is redundant. Remove it; stone continues to pull it in
as needed.

Redoes #517 on top of current main.

Author: AndreyVMarkelov

requirements.txt

@@ -2,7 +2,6 @@
 requests>=2.16.2
 stone>=3.4.0,<4
 # Other dependencies for development
-ply
 pytest
 sphinx
 sphinx_rtd_theme