Fix pytest dependency conflict

[philherbert]

Value Statement

As a VANotify developer
I want to ensure that my dependencies are up to date
So that I can make use of new features and security patches

Acceptance Criteria

WHEN I run pip install -r requirements_for_text.txt
THEN I don't get any dependency conflicts

Checklist

• Checklist item here

Assumptions

Additional Info/Resources

Running pip install -r requirements_for_text.txt without --use-deprecated=legacy-resolver returns the following error:

ERROR: Cannot install pytest-cov==2.6.1, pytest-env==0.6.2, pytest-flask-sqlalchemy==1.0.2, pytest-mock==3.3.1 and pytest==3.10.1 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested pytest==3.10.1
    pytest-cov 2.6.1 depends on pytest>=3.6
    pytest-env 0.6.2 depends on pytest>=2.6.0
    pytest-flask-sqlalchemy 1.0.2 depends on pytest>=3.2.1
    pytest-mock 3.3.1 depends on pytest>=5.0

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/user_guide/#fixing-conflicting-dependencies

To resolve this error, we have to upgrade pytest to 5.0. However, doing this breaks a looot of tests - for example, tests.app.service.test_rest.test_get_detailed_service:

FAILED                    [ 48%]
tests/app/service/test_rest.py:1800 (test_get_detailed_service[today])
Fixture "sample_notification" called directly. Fixtures are not meant to be called directly,
but are created automatically when test functions request them as parameters.
See https://docs.pytest.org/en/latest/fixture.html for more information about fixtures, and
https://docs.pytest.org/en/latest/deprecations.html#calling-fixtures-directly about how to update your code.

See this documentation for a description of the deprecated behaviour.

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