volare/manage: skip directory when extracting

[proppy]

enable currently fails with the following errors if a tar.xz file contains directories:

Traceback (most recent call last):
  File "/tmp/5/env/bin/volare", line 33, in <module>
    sys.exit(load_entry_point('volare', 'console_scripts', 'volare')())
  File "/tmp/5/env/lib/python3.10/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/tmp/5/env/lib/python3.10/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/tmp/5/env/lib/python3.10/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/tmp/5/env/lib/python3.10/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/tmp/5/env/lib/python3.10/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/home/proppy/src/github.com/efabless/volare/volare/manage.py", line 323, in enable_cmd
    enable(pdk_root=pdk_root, pdk=pdk, version=version)
  File "/home/proppy/src/github.com/efabless/volare/volare/manage.py", line 263, in enable
    with tf.extractfile(file) as io:
AttributeError: __enter__

calling extractfile on a TarInfo that's not a regular file will return None preventing the returned object to implement the contextmanager protocol.

This change checks that return TarInfo are regular files before attempting to extract them.