Pin urllib3 to <2.0.0 for python <=3.8 only

[sentrivana]

Following the suggestion here #2070 (comment) this PR pins urllib3 to <2.0.0 only for older Python versions (<=3.8).

TODOs:

• go through the changelog to see what might potentially affect us
• run the tests looking for DeprecationWarnings (see https://urllib3.readthedocs.io/en/latest/v2-migration-guide.html#migrating-as-a-package-maintainer)
• look at actual code changes in the implementation of the things we're using (e.g. PoolManager, ProxyManager)
• play around with the SDK manually

#2070