Ensure updated pygments installed

[ukclivecox]

What this PR does / why we need it:

Fix build failure on readthedocs

Which issue(s) this PR fixes:

Fixes #2455

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[seldondev]

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[seldondev]

Fri Sep 18 17:10:44 UTC 2020
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2457/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2457 --build=1

[seldondev]

Fri Sep 18 17:11:00 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2457/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2457 --build=2