Install package on rtd (numpy#472)

Author: jarrodmillman

.readthedocs.yaml

@@ -28,4 +28,7 @@ sphinx:
 # See https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
 python:
   install:
-    - requirements: requirements/doc.txt
+    - method: pip
+      path: .
+      extra_requirements:
+        - doc

setup.py

@@ -65,6 +65,11 @@ def read(fname):
             for req in read("requirements/test.txt").split("\n")
             if not req.startswith("#")
         ],
+        "doc": [
+            req
+            for req in read("requirements/doc.txt").split("\n")
+            if not req.startswith("#")
+        ],
     },
     package_data={
         "numpydoc": [