fix: do not exceed lifetime of access token in cache

[pvbouwel]

No description provided.

[pvbouwel]

Test failure is unrelated to this PR but should be solved with: #477

[soxofaan]

Maybe you also want to add/edit a test on TestClientCredentialsAccessTokenHelper here?

[soxofaan]

and you probably want to bump the openeo dependency version in setup.py too (because you depend on new API)

[soxofaan]

and merge master in this feature branch to fix the failing test

[pvbouwel]

Maybe you also want to add/edit a test on TestClientCredentialsAccessTokenHelper here?

Moved them to this PR

[pvbouwel]

@soxofaan does it make sense to have _expiration_threshold as an attribute set with default value through initialiser like in my commit?

[soxofaan]

@soxofaan does it make sense to have _expiration_threshold as an attribute set with default value through initialiser like in my commit?

I'm fine with that.

minor nitpick: calling it buffer instead of threshold might be a bit more self-explanatory

[soxofaan]

merged in 8165e0c (with minor fixup)