Fix clearing EnvironmentModifications with python2 (#10791)

* Fix clearing EnvironmentModifications with python2

* Add EnvironmentModifications::clear unit test

Use re-assignment rather than del to clear array

* Fix flake issues

lib/spack/spack/test/environment_modifications.py

@@ -306,3 +306,10 @@ def test_preserve_environment(prepare_environment_for_tests):
     assert 'NOT_SET' not in os.environ
     assert os.environ['UNSET_ME'] == 'foo'
     assert os.environ['PATH_LIST'] == '/path/second:/path/third'
+
+
+def test_clear(env):
+    env.set('A', 'dummy value')
+    assert len(env) > 0
+    env.clear()
+    assert len(env) == 0

lib/spack/spack/util/environment.py

@@ -353,7 +353,7 @@ def clear(self):
         """
         Clears the current list of modifications
         """
-        self.env_modifications.clear()
+        self.env_modifications = []
 
     def apply_modifications(self):
         """Applies the modifications and clears the list."""