Update requirements.txt

[gsusrafael]

Fixing this issue when performing a rebuild:

ERROR: Could not find a version that satisfies the requirement dropbox==11.36.0 (from geonode) (from versions: 1.1, 1.2, 1.3, 1.4, 1.5.1, 1.6, 2.0.0, 2.1.0, 2.2.0, 3.2, 3.12, 3.13, 3.14, 3.21, 3.22, 3.23, 3.24, 3.25, 3.26, 3.27, 3.28, 3.29, 3.30, 3.31, 3.32, 3.34, 3.36, 3.37, 3.38, 3.39, 3.40, 3.41, 3.42, 4.0, 5.0, 5.0.1, 5.1, 5.2, 5.2.1, 5.2.2, 6.0, 6.1, 6.2, 6.3.0, 6.4.0, 6.5.0, 6.6.0, 6.6.1, 6.6.2, 6.7.0, 6.8.0, 6.9.0, 7.1.0, 7.1.1, 7.2.0, 7.2.1, 7.3.0, 7.3.1, 8.0.0, 8.1.0, 8.2.0, 8.3.1, 8.4.0, 8.4.1, 8.5.0, 8.5.1, 8.6.0, 8.7.0, 8.7.1, 8.8.0, 8.8.1, 8.9.0, 9.0.0, 9.1.0, 9.2.0, 9.3.0, 9.4.0, 9.5.0, 10.0.0, 10.1.0, 10.1.1, 10.1.2, 10.2.0, 10.3.0, 10.3.1, 10.4.1, 10.5.0, 10.6.0, 10.7.0, 10.8.0, 10.9.0, 10.10.0, 11.0.0, 11.1.0, 11.2.0, 11.4.0, 11.4.1, 11.5.0, 11.6.0, 11.7.0, 11.8.0, 11.9.0, 11.10.0, 11.11.0, 11.12.0, 11.13.0, 11.13.1, 11.13.2, 11.13.3, 11.14.0, 11.15.0, 11.16.0, 11.17.0, 11.18.0, 11.19.0, 11.20.0, 11.21.0, 11.22.0, 11.23.0, 11.24.0, 11.25.0, 11.26.0, 11.27.0, 11.28.0, 11.29.0, 11.30.0, 11.31.0, 11.32.0, 11.33.0, 11.34.0, 11.35.0, 11.36.0, 11.36.2, 12.0.1, 12.0.2)
322.4 ERROR: No matching distribution found for dropbox==11.36.0

Checklist

Reviewing is a process done by project maintainers, mostly on a volunteer basis. We try to keep the overhead as small as possible and appreciate if you help us to do so by completing the following items. Feel free to ask in a comment if you have troubles with any of them.

For all pull requests:

• Confirm you have read the contribution guidelines
• You have sent a Contribution Licence Agreement (CLA) as necessary (not required for small changes, e.g., fixing typos in the documentation)
• Make sure the first PR targets the master branch, eventual backports will be managed later. This can be ignored if the PR is fixing an issue that only happens in a specific branch, but not in newer ones.

The following are required only for core and extension modules (they are welcomed, but not required, for contrib modules):

• There is a ticket in https://github.com/GeoNode/geonode/issues describing the issue/improvement/feature (a notable exemption is, changes not visible to end-users)
• The issue connected to the PR must have Labels and Milestone assigned
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• PR title must be in the form "[Fixes #<issue_number>] Title of the PR"
• New unit tests have been added covering the changes, unless there is an explanation on why the tests are not necessary/implemented
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• This PR passes the QA checks: black geonode && flake8 geonode
• Commits changing the settings, UI, existing user workflows, or adding new functionality, need to include documentation updates
• Commits adding new texts do use gettext and have updated .po / .mo files (without location infos)

Submitting the PR does not require you to check all items, but by the time it gets merged, they should be either satisfied or inapplicable.

[cla-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have the users @gsusrafael on file. In order for us to review and merge your code, please contact the project maintainers to get yourself added.

[giohappy]

@gsusrafael 4.1.x is not maintained anymore. Anyway, we might merge your PR when tests pass.