improve test coverage

[matthewhegarty]

Problem

For inclusion in Release 3

Code coverage seems to miss certain paths, meaning coverage is lower than it should be.
This is because the IMPORT_EXPORT_TEST_TYPE env var is not being set correctly for test runs, which means that postgres / mysql specific tests are not being run as part of the CI process.

Solution

I switched the CI script to use tox instead of the github actions matrix. This is a better approach because tox is already present in the project, though not being used during CI.

• Fixed environment variables in the CI scripts.
• Added extra unit tests to cover lines being missed in test coverage.
• Removed default username and password from MySQL test db for security best practices (settings.py).
• Added deprecation warning to unused exceptions.py module (and added appropriate test)
• Removed testing against tablibdev for every test run - this seems overkill - so now there is a single test for tablib dev for each test run.
• Moved 'tests/bulk/docker*' to tests
• Updated runtests.sh - now will run tox tests for each supported db (sqlite, postgres, MySQL)
• Added new documentation page testing.rst
• Added MySQL to docker-compose.yml to support testing
• CI builds are also around 5x faster 😺
• Code coverage 100% 🚀

Acceptance Criteria

• Check that Postgres / MySQL specific tests are run (visible in the test output).
• Test coverage increases.

[coveralls]

Coverage increased (+0.2%) to 98.309% when pulling 76fe540 on matthewhegarty:test-coverage-update into 0d4340f on django-import-export:main.

[matthewhegarty]

@manelclos many thanks for reviewing - I've added some comments and made some changes

[manelclos]

LGTM