Fix for Redis / mongo/memcache/filesystem 
Not for sql
Do not save session on each request if session.modified = False

- Marked previously failing tests as SKIP as fixing them is outside the
  scope of this issue.
- Updated pyproject.toml to be able to run the tests
- Updated github actions to run tests

- Tests moved to tests directory
- Testing done via pytest
- Created requirements/pytest.txt for running pytest
- Main code changes based on review

newline

add should_set_cookie check before save new cookies

Version

Previous lack of SECRET_KEY with SESSION_USE_SIGNER=True was:
AttributeError: 'NoneType' object has no attribute 'sign'

Now it is:
KeyError: 'SECRET_KEY must be set when SESSION_USE_SIGNER=True'

Closed #189

Closes #203
Based on flask-session2 rather than other PRs as the expected behaviour would be to remove the session with no expiry

save_session method relies on checking if a session is empty to return early. This fixes the boolean evaluation of a permanent but empty session evaluating as True.

Closes #193

Expect KeyError

Use secrets module for session ids

Pymongo fixes from flask-session2 and #183 #199 #136

fix

Closes #201 #176 #102 #57 #12

Fix bare except statement