Prepare docker alpine to 3.15

[mmacata]

The Dockerfiles will be updated in a separate PR. This here is needed for builds of actinia-docker.

• update requirements.txt: for google-cloud-bigquery a downgrade is necessary, see New dependency on pyarrow introduces heavyweight numpy sub-dependency googleapis/python-bigquery#1196, otherwise pip3 install -r requirements.txt fails while installing new dependency pyarrow.
• cleanup requirements.txt
• for Flask to not require a broken itsdangerous, also a downgrade is necessary, see https://flask.palletsprojects.com/en/2.1.x/changes/#version-2-0-0 and as this is only a solution to progress here, Deprecated usage of python package in user.py #345 was opened.
• The tests fail for new pyproj but will succeed again with alpine update 3.15 as proj 8.2.0 is included there.
• hint to scikit-learn and where it is installed was added