Fix docs build on RTD

requirements/dev.txt

@@ -89,9 +89,6 @@ more-itertools==7.0.0 \
 atomicwrites==1.3.0 \
     --hash=sha256:03472c30eb2c5d1ba9227e4c2ca66ab8287fbfbbda3888aa93dc2e28fc6811b4 \
     --hash=sha256:75a9445bac02d8d058d5e1fe689654ba5a6556a1dfd8ce6ec55a0ed79866cfa6
-attrs==19.1.0 \
-    --hash=sha256:69c0dbf2ed392de1cb5ec704444b08a5ef81680a61cb899dc08127123af36a79 \
-    --hash=sha256:f0b870f674851ecbfbbbd364d6b5cbdff9dcedbc7f3f5e18a6891057f21fe399
 pluggy==0.13.1 \
     --hash=sha256:15b2acde666561e1298d71b523007ed7364de07029219b604cf808bfa1c765b0 \
     --hash=sha256:966c145cd83c96502c3c3868f50408687b38434af77734af1e9ca461a4081d2d

requirements/docs.txt

@@ -180,3 +180,6 @@ typing-extensions==3.10.0.2 \
     --hash=sha256:49f75d16ff11f1cd258e1b988ccff82a3ca5570217d7ad8c5f48205dd99a677e \
     --hash=sha256:d8226d10bc02a29bcc81df19a26e56a9647f8b0a6d4a83924139f4a8b01f17b7 \
     --hash=sha256:f1d25edafde516b146ecd0613dabcc61409817af4766fbbcfb8d1ad4ec441a34
+attrs==19.1.0 \
+    --hash=sha256:69c0dbf2ed392de1cb5ec704444b08a5ef81680a61cb899dc08127123af36a79 \
+    --hash=sha256:f0b870f674851ecbfbbbd364d6b5cbdff9dcedbc7f3f5e18a6891057f21fe399