Add support for python 3.9

[palango]

What this PR does

Signal support for Python 3.9

Why I'm making this PR

What's tricky about this PR (if any)

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Any reviewer can check these:

• If the PR is fixing a bug or adding a feature, add an entry to CHANGELOG.md.
• If the PR changed a Solidity source, run make compile_contracts and add the resulting raiden_contracts/data/contracts.json in the PR.
• If the PR is changing documentation only, add [skip ci] in the commit message so Travis does not waste time.
  • But, if the PR changes comments in a Solidity source, do not add [skip ci] and let Travis check the hash of the source.
• In Python, use keyword arguments
• Squash unnecessary commits
• Comment commits
• Follow naming conventions
  • solidityFunction
  • _solidity_argument
  • solidity_variable
  • python_variable
  • PYTHON_CONSTANT
• Follow the Signature Convention in CONTRIBUTING.md
• For each new contract
  • The deployment script deploys the new contract.
  • etherscan_verify.py runs on the new contract.
• Bookkeep
  • The gas cost of new functions are stored in gas.json.
• Solidity specific conventions
  • Document arguments of functions in natspec
  • Care reentrancy problems
  • if the PR adds or removes require() or assert()
    • add an entry in Changelog
    • open an issue in the client, light client, service repos so the change is reflected there
    • Just adding a message in require() doesn't require these steps.
• When you catch a require() failure in Solidity, look for a specific error message like pytest.raises(TransactionFailed, match="error message"):

And before "merge" all checkboxes have to be checked. If you find redundant points, remove them.

[codecov]

Codecov Report

Merging #1456 (0d8dcfc) into master (24f231d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1456   +/-   ##
=======================================
  Coverage   80.65%   80.65%           
=======================================
  Files          22       22           
  Lines        1561     1561           
  Branches      190      190           
=======================================
  Hits         1259     1259           
  Misses        255      255           
  Partials       47       47           

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