mysql: fix issue when using old core API call (#41573)

MySQL was performing a core API call to `Spec.flat_dependencies`
when setting up the build environment. This function is an
implementation detail of the old concretizer, where multiple nodes
from the same package are not allowed.

This PR uses a more idiomatic way to check if "python" is
in the DAG.

For reference, see #11356 to check why the call was introduced.

lib/spack/spack/spec.py

@@ -3128,9 +3128,7 @@ def flat_dependencies(self, **kwargs):
 
         except spack.error.UnsatisfiableSpecError as e:
             # Here, the DAG contains two instances of the same package
-            # with inconsistent constraints.  Users cannot produce
-            # inconsistent specs like this on the command line: the
-            # parser doesn't allow it. Spack must be broken!
+            # with inconsistent constraints.
             raise InconsistentSpecError("Invalid Spec DAG: %s" % e.message) from e
 
     def index(self, deptype="all"):