docs: separate out dependencies

The intention is to separate "stuff that's needed by this package" from
"stuff that's needed to build the docs".

Change-Id: I7baf27e6f814880ea241ccbd3de4b87a6e7c4d1b

.readthedocs.yml

@@ -2,3 +2,4 @@ build:
     image: latest
 python:
     version: 3.8
+requirements_file: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1,6 @@
+alabaster>=0.7.12,<1
+docutils>=0.15.2,<1
+Pygments>=2.7.4,<3
+rstcheck
+Sphinx>=3.5.0,<4
+sphinxcontrib-bibtex>=0.4.2,<1

requirements.txt

@@ -1,12 +1,7 @@
-alabaster>=0.7.12,<1
-docutils==0.15.2
 matplotlib>=3.3.3,<4
 networkx>=2.5,<3
 numpy>=1.19.4,<2
 pandas>=1.1.5,<2
 pbr>=5.5.1,<6
-Pygments>=2.7.4,<3
 scipy>=1.5.4,<2
-Sphinx>=3.5.0,<4
-sphinxcontrib-bibtex>=0.4.2,<1
 xlrd>=1.2.0,<2

tox.ini

@@ -22,7 +22,7 @@ commands =
 
 [testenv:docs]
 deps =
-	rstcheck
+	-r{toxinidir}/docs/requirements.txt
 whitelist_externals =
 	/bin/sh
 commands =