🚸 slightly tweak how optional qiskit dependency is handled

[burgholzer]

Description

This PR slightly adjusts how the optional qiskit dependency is handled in the load method.
At that point in the function, the user is passing in a Qiskit QuantumCircuit anyway, so Qiskit will naturally be available.

An observation I had while working on this:
We currently do not enforce a particular version constraint on qiskit at runtime.
Thus, someone might use a version of qiskit that is too old at runtime, and will probably run into errors.
We could probably do a better job of suggesting people to install the mqt.core[qiskit] extra to guarantee compatibility.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.2%. Comparing base (a703232) to head (cd94730).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #797     +/-   ##
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- Coverage   92.2%   92.2%   -0.1%     
=======================================
  Files        125     125             
  Lines      13764   13760      -4     
  Branches    2167    2167             
=======================================
- Hits       12701   12697      -4     
  Misses      1063    1063             

Flag	Coverage Δ		*Carryforward flag
cpp	92.0% <ø> (ø)		Carriedforward from a703232
python	99.7% <100.0%> (-0.1%)	⬇️

*This pull request uses carry forward flags. Click here to find out more.

Files with missing lines	Coverage Δ
src/mqt/core/__init__.py	100.0% <100.0%> (ø)
src/mqt/core/plugins/qiskit.py	100.0% <100.0%> (ø)