Explicitly add extras to docs requirements.

Author: domdfcoding

doc-source/requirements.txt

@@ -3,6 +3,9 @@ default-values>=0.4.2
 domdf-sphinx-theme>=0.3.0
 extras-require>=0.2.0
 pandas>=1.1.2
+pytest>=6.2.0
+pytest-regressions>=2.0.2
+pytz>=2019.1
 seed-intersphinx-mapping>=0.3.1
 sphinx<3.4.0,>=3.0.3
 sphinx-autofixture>=0.2.1