Btest

* CI: remove explicit use of Python 3.5, EOL (Christian Kreibich, Corelight)

* Explain recent alternatives-related changes in the README. (Christian Kreibich, Corelight)

* Fix handling of specific alternatives in combination with the default (Christian Kreibich, Corelight)

* Fail when -a/--alternative includes an alternative not defined in the config (Christian Kreibich, Corelight)

* Add test for use of an undefined alternative (Christian Kreibich, Corelight)

* Always use UTF-8 encoding for BTest input and output files.
(Benjamin Bannier, Corelight)

* Prevent multiprocessing state leaks into toplevel .tmp directory (Benjmain
Bannier, Corelight)

* Check whether post-testing .tmp is empty (Benjamin Bannier and Christian
Kreibich, Corelight)

* Clean up registered temp files in btest-rst-cmd (Christian Kreibich,
Corelight)

* Add temp directory to the statefile-sorted test (Christian Kreibich,
Corelight)

* Remove use of Python 3.7 feature `time.time_ns`. (Benjamin
Bannier, Corelight)

* Use appropiate Sphinx versions for different Python versions.
(Benjamin Bannier, Corelight)

* GH-66: Fix typo preventing use of configured Python version in CI.
(Benjamin Bannier, Corelight)

* Remove uses of deprecated `distutils` package. (Benjamin Bannier, Corelight)

* Improve backtick expansion in `btest.cfg`. Instead of naive
splitting of the command string into arguments, pass the command
as-is to a subshell for parsing. Note that this changes semantics
of backticks slightly. However, they were ill-defined to begin
with. (DJ Gregor, Corelight)

* Minor README tweaks. (DJ Gregor, Corelight)