zstandard: vendor python-zstandard 0.11 The upstream source distribution from PyPI was extracted. Unwanted files were removed. The clang-format ignore list was updated to reflect the new source of files. The project contains a vendored copy of zstandard 1.3.8. The old version was 1.3.6. This should result in some minor performance wins. test-check-py3-compat.t was updated to reflect now-passing tests on Python 3.8. Some HTTP tests were updated to reflect new zstd compression output. # no-check-commit because 3rd party code has different style guidelines Differential Revision: https://phab.mercurial-scm.org/D6199