Bote

* Marked as compatible with Python 3.10 as tests with release candidate 1 run flawlessly on Linux, MacOS, and Windows.
* Update dependencies to versions compatible with Python 3.10.

* Use [custom exceptions](bote/err.py).
* Run tests for Python 3.10 with Beta 4 instead of Beta 3.

* Small code improvements.
* Updated dependencies.
* Impoved code testing:
* Tests now also run with Python `3.10.0-beta.3` on Ubuntu.
* Although the code should be platform independent, tests are now also run with MacOS and Windows VMs to be sure.
* Improved test coverage from 85 to 97%.

* Make compatible with the newest version of its sister project `userprovided`. (Older versions still work.)
* The optional parameter `wrap_width` allows you to set after how many characters a line is wrapped. (Defaults to 80).
* The parameter `recipient` can now be either a string or a dictionary. If it is a string, that address will be used as a recipient as long that is not overwritten. In case you use a dictionary, the behavior is the same if you defined a key named `default`. Defining an additional `admin` key allows you to use the new `send_mail_to_admin` command.

* Change development status from `beta` to `stable`.
* Check used Python version with the [`compatibility`](https://github.com/RuedigerVoigt/compatibility) package. (A sister-project so development is synchronized.)
* Replaced `unittest` with the `pytest` testing framework and increased test coverage.
* Fixed minor bug: Would not raise an exception if no port was provided for an external server

* Now also test with Python 3.9.