Proofpoint TAP: Bugfix / Python3

[dfederschmidt]

Pull Request Checklist

Please check if your PR fulfills the following requirements:

• [x] Testing of all the changes has been performed (for bug fixes / features)
• [x] The readme.html has been reviewed and added / updated if needed (for bug fixes / features)
• [x] Use the following format for the PR description: <App Name>: <PR Type> - <PR Description>
• [x] Provide release notes as part of the PR submission which describe high level points about the changes for the upcoming GA release.
• [ ] Verify all checks are passing.
• [ ] Do NOT use the next branch of the forked repo. Create separate feature branch for raising the PR.
• [ ] Do NOT submit updates to dependencies unless it fixes an issue.

  Pull Request Type

Please check the type of change your PR introduces:

• [ ] New App
• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [x] Refactoring (no functional changes, no api changes)
• [ ] Documentation
• [ ] Other (please describe):

Release Notes (REQUIRED)

• Migration to Python 3
• Bugfix for the Polling action

[splunk-soar-connectors-bot]

@dfederschmidt Static Tests results for commit 7c9d600777132dc1b3ad839a80a71f067bf7000e - https://drive.google.com/file/d/1O0Yt3W38arQl5jO8SqQaH_3D7lQrdmOM/view?usp=drivesdk

[splunk-soar-connectors-bot]

@dfederschmidt Compile Tests results for commit 7c9d600777132dc1b3ad839a80a71f067bf7000e - https://drive.google.com/file/d/19hriIX4NIXBdvd3HNGSKs4pZDg3p3Dqm/view?usp=drivesdk

[mmomin-crest]

Added this PR into https://github.com/splunk-soar-connectors/proofpoint/pull/5 PR. Hence, Closing this PR.