itsdangerous 2.1.0 removed JSONWebSignatureSerializer

[tobixx]

File "/usr/local/lib/python3.9/site-packages/flask_oidc/__init__.py", line 41, in <module>
ImportError: cannot import name 'JSONWebSignatureSerializer' from 'itsdangerous' (/usr/local/lib/python3.9/site-packages/itsdangerous/__init__.py)

ref https://github.com/pallets/itsdangerous/pull/273/files#diff-da5963cc4161439382e422e8936175c7b46f6d4dc908e2a5d344e697dc77ad0a

quickfix: pin requirement to itsdangerous-2.0.1 until better solution

[tobixx]

fix in #141