Add pip check to CI workflows

[csala]

The current CI workflows and local test and lint commands do not catch dependency incompatibilities.

For example, installing the repository for development on the v0.6.0-dev branch results in these errors:

ERROR: flake8 3.9.2 has requirement pycodestyle<2.8.0,>=2.7.0, but you'll have pycodestyle 2.8.0 which is incompatible.
ERROR: flake8 3.9.2 has requirement pyflakes<2.4.0,>=2.3.0, but you'll have pyflakes 2.4.0 which is incompatible.

A pip check command should be made part of the local and CI tests to make sure that our dependency tree is always clean.