[BUG] Docs are not automatically building with newest MarkDown content

[ysuarez]

What steps does it take to reproduce the issue?

Visit this official docs page and see that the changes from this merged PR are not reflected

Here is a link that displays errors in building docs.
https://github.com/Islandora/documentation/actions

• When does this issue occur?

n/a

• Which page does it occur on?

In a few official doc pages that should have newer content, for example https://islandora.github.io/documentation/installation/docker-maintain-drupal/

• What happens?
  n/a

• To whom does it occur (anonymous visitor, editor, administrator)?

All visitors to the official docs.

• What did you expect to happen?

That the latest documentation commits would lead to new markdown content.

Which version of Islandora are you using?

Any related open or closed issues to this bug report?

Screenshots:

[rosiel]

Found the source: mkdocs-material-insiders, our theme, requires pymdown-extensions>=9.4 , but pymdown-extensions dropped python 3.6 support back in 9.2. Our Travis environment runs on python3.6.7, which seems to be a default. We might be able to specify the python version in the Travis file to something more recent.

[rosiel]

Resolved with #2115