Pyqmix

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* The DLL search improvement introduced in 2021.1 is now working correctly for
Python 3.6 and 3.7, too.

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* Improve automated DLL search path configuration; the DLLs should be loadable
out-of-the-box in more situations now, including on systems running Python
3.8 and newer.
* Switch back to using `pywin32` instead of `pypiwin32`.

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- Get rid of potential caching problems by disabling offline functionality of
the frontend (i.e. we have disabled the service worker)
- Update `react` and `react-dom` to 16.9.0, `react-scripts` to 3.0.0,
`bootstrap` to 4.3.1, and `reactstrap` to 8.0.1
- Enable running pyqmix-web as an entry-point
- Fix a minor (display) bug in the frontend
- Improve developers' documentation.
- Frontend now looks for all assets placed relative to "index.html"
- The frontend title is now fixed on top and hovering above all other
content while scrolling
- Refactor many parts of the pump form into separate React Components
- Disable input fields until at least one pump is selected
- The "Disconnect" button is now displayed in a dangerous red
- Backend test mode can now be enabled by setting the environment variable
`PYQMIX_TEST_MODE`.
- Display copyright notice in frontend

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* Handle non-existent configuration directory

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* Update installation instructions
* Automated testing using Travis now also runs on Python 2.7 (only tested
Python 3 before)
* `switch_valve_when_done` keyword argument is now available for all pumping
operations
* Depend on `pypiwin32` instead of `pywin32`

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* Report correct version number when installed from wheel.