Use proper python version in CI

Apparently CI has been completely broken I don't know how long due to
pipx using the python interpreter it was installed for, instead of using
the one we specifically prepared for tests. This patch uses python3
alias to install tox and run tests, which must guarantee proper python
interpreter.

Author: ikalnytskyi

.github/workflows/ci.yml

@@ -19,7 +19,9 @@ jobs:
           python-version: "3.12"
 
       - name: Run ruff
-        run: pipx run tox -e lint
+        run: |
+          python3 -m pip install tox
+          python3 -m tox -e lint
         env:
           RUFF_OUTPUT_FORMAT: github
 
@@ -53,4 +55,6 @@ jobs:
         if: matrix.os == 'macos-latest'
 
       - name: Run pytest
-        run: pipx run tox -e test
+        run: |
+          python3 -m pip install tox
+          python3 -m tox -e test