Fix cryptography build in travis cicd by upgrading distribution from …

[wind-chh]

…Trusty Tahr to Focal Fossa (pdfminer#585)

• Update .travis.yml

• Also change 3.9-dev to 3.9 because that is now supported by travis

Pull request

Thanks for improving pdfminer.six! Please include the following information to
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• A description of why this PR is needed. What does it fix? What does it
  improve?
• A summary of the things that this PR changes.
• Reference the issues that this PR fixes (use the fixes #(issue nr) syntax).
  If this PR does not fix any issue, create the issue first and mention that
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How Has This Been Tested?

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Checklist

• I have added tests that prove my fix is effective or that my feature
  works
• I have added docstrings to newly created methods and classes
• I have optimized the code at least one time after creating the initial
  version
• I have updated the README.md or I am verified that this
  is not necessary
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  CHANGELOG.md