Fix broken CI for package only PRs, make dateutil not strictly required (#24484)

* Force the Python interpreter with an env variable

This commit forces the Python interpreter with an
environment variable, to ensure that the Python set
by the "setup-python" action is the one being used.

Due to the policy adopted by Spack to prefer python3
over python we may end up picking a Python 3.X
interpreter where Python 2.7 was meant to be used.

* Revert "Update conftest.py (#24473)"

This reverts commit 477c8ce820.

* Make python-dateutil a soft dependency for unit tests

Before #23212 people could clone spack and run
```
spack unit-tests
```
while now this is not possible, since python-dateutil is
a required but not vendored dependency. This change makes
it not a hard requirement, i.e. it will be used if found
in the current interpreter.

* Workaround mypy complaint

.github/workflows/unit_tests.yaml

@@ -156,6 +156,8 @@ jobs:
           make -C ${KCOV_ROOT}/build && sudo  make -C ${KCOV_ROOT}/build install
     - name: Bootstrap clingo from sources
       if: ${{ matrix.concretizer == 'clingo' }}
+      env:
+          SPACK_PYTHON: python
       run: |
           . share/spack/setup-env.sh
           spack external find --not-buildable cmake bison
@@ -163,6 +165,7 @@ jobs:
     - name: Run unit tests (full suite with coverage)
       if: ${{ needs.changes.outputs.with_coverage == 'true' }}
       env:
+          SPACK_PYTHON: python
           COVERAGE: true
           SPACK_TEST_SOLVER: ${{ matrix.concretizer }}
       run: |
@@ -172,6 +175,7 @@ jobs:
     - name: Run unit tests (reduced suite without coverage)
       if: ${{ needs.changes.outputs.with_coverage == 'false' }}
       env:
+          SPACK_PYTHON: python
           ONLY_PACKAGES: true
           SPACK_TEST_SOLVER: ${{ matrix.concretizer }}
       run: |