Unit tests no longer work

[LeXofLeviafan]

As of now, unit tests run by tox are outright failing due to incompatibilities between dependency libraries (caused by recently released new versions of some of them).

To fix this, the files setup.py, requirements.txt & bukuserver/requirements.txt need to be updated with the following changes, at least for the time being:

• Flask>=2.2.2,<2.3 (2.3 has json_encoder property missing, which causes HTTP 500 on several queries)
• urllib3>=1.23,<2 (2.0 doesn't have VerifiedHTTPSConnection, which breaks pytest itself of all things)

[jarun]

Please raise the PR.