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 477c8ce8205ec149fa897c9d83e530815c978d8b.

* 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

0 files changed, 0 insertions, 0 deletions