Backport PR #41092: CI/DOC: temporary pin environment to python 3.8 (#41095)

Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>

Author: simonjayhawkins

environment.yml

@@ -4,7 +4,7 @@ channels:
 dependencies:
   # required
   - numpy>=1.16.5
-  - python=3
+  - python=3.8
   - python-dateutil>=2.7.3
   - pytz