Add pytest-mpl dependency
Explanation of changes
This change is needed for ci!20 (merged) (and therefore quantify-scheduler!1071 (merged) indirectly), as without it, the test job won't understand the --mpl option and give an error.
Merge checklist
See also merge request guidelines
- 
Merge request has been reviewed (in-depth by a knowledgeable contributor), and is approved by a project maintainer.  - 
New code is covered by unit tests (or N/A).  - 
New code is documented and docstrings use numpydoc format (or N/A).  - 
New functionality: considered making private instead of extending public API (or N/A).  - 
Public API changed: added @deprecated(or N/A). - 
Newly added/adjusted documentation and docstrings render properly (or N/A).  - 
Pipeline fix or dependency update: post in #software-for-developerschannel to mergemainback in or update local packages (or N/A). - 
Tested on hardware (or N/A).  - 
CHANGELOG.mdandAUTHORS.mdhave been updated (or N/A). - 
Windows tests in CI pipeline pass (manually triggered by maintainers before merging). - Maintainers do not hit Auto-merge, we need to actively check as manual tests do not block pipeline
 
 
For reference, the issues workflow is described in the contribution guidelines.
Edited  by Thomas Middelburg