Regression from #62: verify_ssl true used to resolve to the system CA
bundle path, so corporate CAs installed in the OS worked; after the
unification true became requests' default (certifi), breaking setups
with a corporate CA in the system store.
Now verify_ssl true injects truststore, so requests verifies against
the OS trust store on any platform. verify_ssl false / custom CA path
behavior is unchanged. Tests mock truststore via an autouse fixture to
keep the pytest process free of global ssl mutation.
Refs #62
3accb1212c