Pinning qiskit in setup.py

[PietropaoloFrisoni]

This PR implements a hard pin on qiskit so that the latter automatically downgrades to qiskit 1.2.4 (that works with this plugin) if the latest version of qiskit (that is, qiskit 1.3.0, which does not currently work with this plugin) is installed.

With this hard pin, if a user has the latest version of qiskit and installs the pennylane-qiskit plugin, the qiskit version is automatically downgraded to the previous one (which does not provide wrong results).

Vice versa, if the user installs the latest pennylane-qiskit and then the latest qiskit version, he/she should receive the message:

pennylane-qiskit 0.40.XXX requires qiskit<1.3.0,>=0.32, but you have qiskit 1.3.0 which is incompatible

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (4742bdb) to head (1b0b516).
Report is 1 commits behind head on master.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            9         9           
  Lines          868       868           
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  Hits           868       868           

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