Pystemd

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* adding coverage information
* changes tests to pystes
* adding support for pyproject.toml
* adding extra properties for unit

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* initial support for pystemd.futures

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* native support for stop_cmd, (pre|post)_start_cmd and post_stop_cmd in pystemd.run.
* adding lxml as a dependency.
* drop support for python 3.4 and 3.5 (yey welcome f-strings)
* change using select.select to use DefaultSelector in pystemd.run

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* support for interactive auth.

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* add initial support for Python 3.9
* include .pxd and .pxi files in the source distribution

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* add initial support for python 3.7 and 3.8
* added options StandardOutputFile, StandardOutputFileToAppend, StandardErrorFile and
StandardErrorFileToAppend to known unit signatures.
* Added type stubs for `pystemd.daemon`, `pystemd.dbuslib`, `pystemd.systemd1.Unit`
and `pystemd.systemd1.Manager` because we still "support python 3.4.
* fix issue when char is unsigned, and <char *> -1 returns 255.
* added `slice_` option to pystemd.run to specify the cgroup where the unit is created.