Create tests for verifying certificate handling wrt. to production dependencies only

[ckunki]

Pyexasol versions <1.0 only had a transitive test dependency to cryptography.

For supporting for newer version of Exasol databases, pyexasol's support for SSL certificates was enhanced, which now also requires dependency cryptography in production.

Hence, pyexasol's tests did not discover the dependency to be missing, see

• Add dependency "cryptography" #245

The current ticket therefore requests adding tests, effectively checking for potentially missing production dependencies.

[ckunki]

Proposal from @tkilias :

• Install only production dependencies
• Execute (some) pyexasol examples

[ArBridgeman]

Should double-check caching mechanism to determine if needed extension in PTB (or to be disabled) when these tests are introduced