Copy docs requirements to a separate file due to RTD constraints. Fixes #2001....

Copy docs requirements to a separate file due to RTD constraints. Fixes #2001. Ref readthedocs/readthedocs.org#6662

.readthedocs.yml

@@ -2,4 +2,5 @@ python:
   version: 3
   extra_requirements:
   - docs
-  pip_install: true
+  pip_install: false
+  requirements: docs/requirements.txt

docs/requirements.txt

+# keep these in sync with setup.cfg
+sphinx
+jaraco.packaging>=6.1
+rst.linker>=1.9

setup.cfg

@@ -74,6 +74,7 @@ tests =
     pip>=19.1 # For proper file:// URLs support.
 
 docs =
+    # Keep these in sync with docs/requirements.txt
     sphinx
     jaraco.packaging>=6.1
     rst.linker>=1.9