archival: abort if compression method is unavailable
`tarfile.CompressionError` is documented to be the "exception for unavailable
compression methods".
Also, make tests conditional on whether the lzma module is available or not.
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true