This fixes the following:
"
/home/jjardon/buildstream/buildstream/doc/bst2html.py:32: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  from collections import Mapping
"

  o Added feature to allow running shell commands

  o Removed unused codepaths to launch bst commands through the script,
    this program is anyway only used to parse and run the session files.

  o Fixed previous patch to run BuildStream uninstalled with
    the subprocess module, as running it in the same interpretor
    causes a bunch of problems.

If --force is not specified, then we'll skip session files in
the case that all of the outputs exist.

Now setting BST_FORCE_SESSION_REBUILD when building the docs
will cause the session files to be rebuilt regardless of whether
they exist or not.

The .gitlab-ci.yml was also changed to use this and force rebuilds.

When specifying a fake-output string, we don't really run the command
or assume it was a `bst` command, and we pretend that `fake-output`
was the output of the command.

Specifying an empty string explicitly enables the behavior too
for faking a command that has no stdout/stderr.

This also adds the "remove-files" hack allowing the session scripts
to remove files before executing commands (kind of unsure if we're
gonna keep this...)