Pytracking

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- Updated django-ipware to >=2.0.
- Updated tests structure and added Github Actions for running tests
- Updated supported Python versions to 3.6, 3.7, 3.8, 3.9

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- Added append_slash configuration parameter to append a slash to click and
open tracking URLs
- Updated dependencies to prevent breaking changes in django-ipware
- Updated supported Python version to 3.6
- Updated README file
- Removed CircleCI integration

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- Added request parameter to notify_decoding_error callback in
``pytracking.django.TrackingView``
- Added a reference to professional services in README.rst

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- First release!
- Support encoding and decoding of open/click tracking links.
- Basic Django Views.
- Can send webhook POST requests with tracking data.
- Can encrypt tracking data instead of relying on base64.
- Can modify HTML content to replace links and add tracking pixel.