Update py-borgbackup to 1.1.17.

Fixes:

- pyinstaller dir-mode: fix pyi detection / LIBPATH treatment
- handle crash due to kill stale lock race
- fix BORG_CACHE_DIR crashing borg if empty
- create --dry-run: fix display of kept tagfile
- fix missing parameter in "did not consistently fail" msg
- missing / healed chunks: always tell chunk ID
- benchmark: make sure cleanup happens even on exceptions

New features:

- implement BORG_SELFTEST env variable.
  this can be used to accelerate borg startup a bit. not recommended for
  normal usage, but borg mass hosters with a lot of borg invocations can
  save some resources with this. on my laptop, this saved ~100ms cpu time
  (sys+user) per borg command invocation.
- implement BORG_LIBC env variable to give the libc filename.
  you can use this if a borg does not find your libc.
- check: add progress indicator for archive check.
- allow --files-cache=size (not recommended, make sure you know what you do)

Other changes:

- Python 3.10 now officially supported!
  we test on py310-dev on github CI since a while and now also on the vagrant
  machines, so it should work ok.
- github CI: test on py310 (again)
- get rid of distutils, use packaging and setuptools.
  distutils is deprecated and gives warnings on py 3.10.
- setup.py: rename "clean" to "clean2" to avoid shadowing the "clean" command.
- remove libc filename fallback for the BSDs (there is no "usual" name)
- cleanup flake8 checks, fix some pep8 violations.
- docs building: replace deprecated function ".add_stylesheet()" for Sphinx 4 compatibility
- docs:

  - add a hint on sleeping computer and ssh connections
  - update the documentation on hacked backup client
  - improve docs/FAQ about append-only remote repos
  - complement the documentation for pattern files and exclude files
  - "filename with spaces" example added to exclude file
    note: no whitespace escaping needed, processed by borg.
  - add info on renaming repositories
  - clarify borg check --verify-data
  - add notice about defective hardware to check documentation
  - add paragraph added in #5855 to utility documentation source
  - add missing leading slashes in help patterns
  - clarify "you will need key and passphrase" borg init warning
  - pull mode: add some warnings
  - mention tar --compare (compare archive to fs files)


(bsiegert)