🔥 Replace weakly maintained setup-nox action (#199)

## Description

The `setup-nox` action (https://github.com/daisylb/setup-nox) isn't very
well maintained. This PR drops it in favor of a simple `pipx install
nox` and the corresponding Python setup.

## Checklist:

<!---
This checklist serves as a reminder of a couple of things that ensure
your pull request will be merged swiftly.
-->

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

Signed-off-by: Lukas Burgholzer <lukas.burgholzer@jku.at>

.github/workflows/python-ci.yml

@@ -21,7 +21,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - name: Setup nox
-        uses: excitedleigh/setup-nox@v2.1.0
+        run: pipx install nox
       - name: Run mypy via nox
         run: nox -s mypy
 
@@ -45,8 +45,12 @@ jobs:
         with:
           submodules: recursive
           fetch-depth: 0
+      - name: Setup Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
       - name: Setup nox
-        uses: excitedleigh/setup-nox@v2.1.0
+        run: pipx install nox
       - name: Test on 🐍 ${{ matrix.python-version }}
         run: nox -s tests-${{ matrix.python-version }}
 
@@ -59,7 +63,7 @@ jobs:
           submodules: recursive
           fetch-depth: 0
       - name: Setup nox
-        uses: excitedleigh/setup-nox@v2.1.0
+        run: pipx install nox
       - name: Run session
         run: nox -s min_qiskit_version -- --cov-report=xml
       - name: Upload Coverage to Codecov
@@ -81,8 +85,12 @@ jobs:
         with:
           submodules: recursive
           fetch-depth: 0
+      - name: Setup Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
       - name: Setup nox
-        uses: excitedleigh/setup-nox@v2.1.0
+        run: pipx install nox
       - name: Run session
         run: nox -s coverage-${{ matrix.python-version }} -- --cov-report=xml
       - name: Upload Coverage to Codecov