Remove support for .tar.bz2 sdists

PEP 527 deprecated .tar.bz2 sdists in September 2016 and PyPI removed
support in April 2020 pypi/warehouse#7529.

There is little evidence of other package indexes supporting this file
format for sdists and there is no known modern Python package build
tool that generates source distributions as .tat.bz2 archives.

Moving to 'packaging' for parsing metadata as the code for extracting
the metadata from distribution archives will need to be implemented in
twine. This change allows to have to implement only one archive format
for sdists.

Note that PyPI still supports sdists in .zip format but twine does not.

Fixes #1196.

twine/package.py

@@ -52,7 +52,6 @@
 
 DIST_EXTENSIONS = {
     ".whl": "bdist_wheel",
-    ".tar.bz2": "sdist",
     ".tar.gz": "sdist",
     ".zip": "sdist",
 }