title: Unknown distribution format: '' when uploading to PyPI via Twine
body: <p>I am trying to update the version of <code>infixpy</code> using <code>twine</code>. Here is my <code>~/.pypirc</code>:</p>
  <pre><code>index-servers =
    pypi
    pypitest
  
  [pypi]
  repository: https://upload.pypi.org/legacy/ 
  username: myuser
  password: mypassword
  
  [pypitest]
  repository: https://upload.testpypi.org/legacy
  username: myuser
  password: mypassword
  </code></pre>
  <p>Here is the command line:</p>
  <pre><code>python setup.py build
  twine upload -r  pypi dist/
  </code></pre>
  <p>The upload errors out with  <code>InvalidDistribution: Unknown distribution format: ''</code></p>
  <p>Here is the full output:</p>
  <pre><code>Processing dependencies for infixpy==0.0.6
  Finished processing dependencies for infixpy==0.0.6
  Processing /Users/steve/git/infixpy
  Building wheels for collected packages: infixpy
    Building wheel for infixpy (setup.py) ... done
    Created wheel for infixpy: filename=infixpy-0.0.6-py3-none-any.whl size=43459 sha256=01fed46f42fa86475079636a55685c93521989aa0ba6558726a9d35c01004b7a
    Stored in directory: /private/var/folders/d6/m67jyndd7h754m3810cl3bpm0000gp/T/pip-ephem-wheel-cache-1bizg6_y/wheels/47/66/74/d79a56979feba04c8ef05e12fe861cacf813cecd397e57071f
  Successfully built infixpy
  Installing collected packages: infixpy
    Attempting uninstall: infixpy
      Found existing installation: infixpy 0.0.6
      Uninstalling infixpy-0.0.6:
        Successfully uninstalled infixpy-0.0.6
  Successfully installed infixpy-0.0.6
  Uploading distributions to https://upload.pypi.org/legacy/
  InvalidDistribution: Unknown distribution format: ''
  </code></pre>
  <p>What corrections are needed to my publishing process? I am on Python 3.7 on macOS.</p>