Chore: pin fastapi and httpx to compatibile versions

[cgraham-rs]

Description

Some Connect rsconnect-python deployment tests failed for fastapitableau which lead me here to try to find a fix.

Addresses #48

Problem

The latest fastapi has an incompatability with fastapitableau

Solution

• Pin to the last know compatible version of fastapi
• Also pin to a known compatibile version of httpx which appears to be used in testing only

Otherwise fastapitableau will need code changes to work with the very latest versions of these dependencies.

Testing Notes / Validation Steps

All tests should pass

pipenv --rm
rm -f Pipfile.lock
pipenv install --dev
just test