- GLPK now respects Configuration.tolerances.integrality again
- removed support for Python 3.5
- added support for Python 3.9
- enabled code coverage in tox
- support symengine 0.7.0
- add [OSQP](https://github.com/oxfordcontrol/osqp) as additional solver
- add [cbc](https://github.com/coin-or/python-mip) as additional solver
- fix: except AttributeError when setting tolerance on cloned Configuration
- enable cloning models between solver interfaces that do not support the same set of tolerance parameters