Implement 553 Version Not Supported

[CasperWA]

Closes #391

New middleware to check whether versioned base URL (relating to the OPTIMADE server) is a valid version.
If it is not a valid version, the middleware will raise, resulting in a 553 Version Not Supported as required by the specification.

Note: This PR changes test_config.json to align the base_url configuration parameter with the used test client's.
This speaks to the fact that the default_config.json and example_config.json should be usable as proper configuration files for development of this package.

This also updates a bit of CI now:
- Use v1 of CasperWA/push-protected action.
- Change job name deps_static to pytest.
- Use a high verbosity for pytest, i.e., -vvv.

Edit: These updates have been factored out into another PR, since they are out-of-scope for this PR. See #422.

[CasperWA]

Don't know why the tests are failing here. Locally they run fine for me.

[CasperWA]

Gah - just realized it's 553 not 533...

[codecov]

Codecov Report

Merging #420 into master will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #420      +/-   ##
==========================================
+ Coverage   90.99%   91.09%   +0.09%     
==========================================
  Files          55       55              
  Lines        2522     2548      +26     
==========================================
+ Hits         2295     2321      +26     
  Misses        227      227              

Flag	Coverage Δ
#unittests	91.09% <100.00%> (+0.09%)	⬆️

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Impacted Files	Coverage Δ
optimade/server/exceptions.py	100.00% <100.00%> (ø)
optimade/server/main.py	95.83% <100.00%> (+0.08%)	⬆️
optimade/server/main_index.py	95.83% <100.00%> (+0.08%)	⬆️
optimade/server/middleware.py	100.00% <100.00%> (ø)

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[CasperWA]

Don't know why the tests are failing here. Locally they run fine for me.

Welp. It seems that maybe the either the pytest_config change in the most top-level conftest.py was not being run for the original CI step that runs tests using mongomock or some of the tests running prior to the tests in said step changes/fixes some things needed for these tests.
I personally think the first one is the case.
If we want to run the original step, I suggest to move to this pytest_config being a pytest fixture instead on the session scope with autouse=True that basically sets the OPTIMADE_CONFIG_FILE environment value (and resets it after all tests have run using try-finally).
However, I'm not sure we need to go back to the original step. The move to a fixture may be needed in any case?

[CasperWA]

When/if this is to be merged, we need to first update the branch protection to use pytest (3.X) instead of deps_static (3.X). But that should be done immediately before this PR is ready to be merged (if we wish to keep in the change of the job name from deps_static to pytest of course).

[CasperWA]

Factored out all out-of-topic changes in this PR and created another PR for them (#422). This means the CI tests for deps_static might fail when getting to run the mongomock pytests.

[CasperWA]

When/if this is to be merged, we need to first update the branch protection to use pytest (3.X) instead of deps_static (3.X). But that should be done immediately before this PR is ready to be merged (if we wish to keep in the change of the job name from deps_static to pytest of course).

This is now not the case for this PR, but rather for #422.

[ml-evs]

Few bits and bobs, mostly to do with testing slashed base URLs without endpoints, otherwise this lgtm

[ml-evs]

👍