fix: add versioneer for rtfd

Midnighter · Midnighter · commit 4af08ad8240e · 2021-01-31T23:14:12.000+01:00
Readthedocs uses `python setup.py install` rather than `pip install`
bypassing many modern installation features.
diff --git a/docs/requirements.txt b/docs/requirements.txt
@@ -2,3 +2,4 @@ mkdocs ~=1.1
 mkdocs-material ~=6.1
 mkdocs-awesome-pages-plugin ~=2.4
 mkdocstrings ~=0.14
+versioneer-518
