Fix urllib3 version to 1.26.15

[dachengx]

What does the code in this PR do / what does it improve?

Because from 2.0.0 urllib3 does not support OpenSSL versions older than 1.1.1. So triggered this failure: https://readthedocs.org/projects/straxen/builds/20440356/. Reference: https://urllib3.readthedocs.io/en/latest/v2-migration-guide.html#what-are-the-important-changes.

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• Update the docstring(s)
• Update the documentation
• Tests to check the (new) code is working as desired.
• Does it solve one of the open issues on github?

Notes on testing

• Until the automated tests pass, please mark the PR as a draft.
• On the XENONnT fork we test with database access, on private forks there is no database access for security considerations.

All italic comments can be removed from this template.

[coveralls]

Coverage: 93.437% (-0.01%) from 93.447% when pulling 2caed52 on fix_urllib3 into 306d98e on master.