[CI] Issue with jinja2==2.11.2 (pinned to jinja2==2.11.1)

[mmcky]

CI has caught an issue with the latest jinja2 bundled with anaconda.

Currently pinning jinja2 to 2.11.1 so templates can be used in previews and check builds

[mmcky]

There is a secondary issue with nbconvert

• unpin anaconda==2020.07 once worked through these issues

[mmcky]

Closing this as lecture-python.myst is building with anaconda=2020.11 so not worth investing to fix this here.