It's sufficient to simply supply the docs/requirements.txt but not specify it.

Jason R. Coombs · Jason R. Coombs · commit fbbedea8f2e9 · 2018-03-12T12:51:02.000-04:00
diff --git a/.readthedocs.yml b/.readthedocs.yml
@@ -1,3 +1,2 @@
-requirements_file: docs/requirements.txt
 python:
   version: 3

Original file line number	Diff line number	Diff line change
`@@ -1,3 +1,2 @@`
`1`		`-requirements_file: docs/requirements.txt`
`2`	`1`	`python:`
`3`	`2`	`version: 3`