tests: removes example app test

[Glignos]

• Pins Sphinx<3 (closes tests: fix build error by removing example app. #199)

Closes #202.

[Glignos]

Passed build

[ppanero]

@Glignos just a minor. Feel free to take it or leave it, I can simply trigger the build when releasing.

[ppanero]

minor: since we have to trigger build here (just to have one passing and not merge force, checked the passed link thanks for that ;)

Can it be changed to:

• Start items with capitals.
• End sentences with dots.
• Expand versions (i.e. 3.0.0).

Mainly to be consistent with the rest of the CHANGES.rst file (yes we need a common one for Invenio 😅 )