Improve wording for incompatible Python versions

poetry/mixology/failure.py

@@ -39,7 +39,7 @@ class _Writer:
 
         if required_python_version is not None:
             buffer.append(
-                "The current supported Python versions are {}".format(
+                "The current project must support the following Python versions: {}".format(
                     required_python_version
                 )
             )

tests/mixology/version_solver/test_python_constraint.py

@@ -8,7 +8,7 @@ def test_dependency_does_not_match_root_python_constraint(root, provider, repo):
 
     add_to_repo(repo, "foo", "1.0.0", python="<3.5")
 
-    error = """The current supported Python versions are ^3.6
+    error = """The current project must support the following Python versions: ^3.6
 
 Because no versions of foo match !=1.0.0
  and foo (1.0.0) requires Python <3.5, foo is forbidden.