Update test matrix to cover missing Flask components

[jcomo]

The test matrix in CI currently uses multiple versions of Flask but does not cover the cases where components (ie. blinker) are installed and then not installed. Because of this, we were unable to catch the issue reported in: https://github.com/jarus/flask-testing/issues/91 but it would have been immediately obvious with the proper facilities.

Investigate a solution such that the build matrix can have different configurations of flask installations