Support low-ulimit platforms in Python runtime

The Python function runtime should raise its soft ulimit for open files
to match the hard limit at startup. Go and Java runtimes already do this
automatically. Without it, the Python runtime fails under load on
platforms with a low default soft limit (e.g. 1024).

The fix belongs in the middleware startup path.

Related: [SRVOCF-788](https://redhat.atlassian.net/browse/SRVOCF-788)

Start in the middleware startup path for the Python function runtime and compare how the Go and Java runtimes handle their open-file limits. Reproduce the low-soft-limit case, then verify that startup raises the soft limit to the hard limit and that the relevant runtime checks pass.

Written by the indexing model from the issue text.