net/py-urllib3: disable SSL by default

The [secure] extra (provided by the SSL option) is deprecated:
https://github.com/urllib3/urllib3/issues/2680
...but the pyOpenSSL bit is un-deprecated:
https://github.com/urllib3/urllib3/issues/3126

Given that even pyOpenSSL is regarded as an optional third-party
module, disable by default. By side effect, reduces build load,
induced by mainline py-cryptography's required Rust code, for those
ports using sphinx to build documentation, as this is part of
sphinx's dependency tree.