Skip tests requiring invoke if it's not installed

Since invoke is an optional dependency and only one group of tests
require it, skip them gracefully rather than failing if it's not
present.

tests/test_config.py

@@ -6,4 +6,5 @@
 
 from paramiko.py3compat import string_types
 
+try:
     from invoke import Result
@@ -9,4 +10,7 @@
     from invoke import Result
+except ImportError:
+    Result = None
+
 from mock import patch
 from pytest import raises, mark, fixture
 
@@ -705,6 +709,7 @@
     return inner
 
 
+@mark.skipif(Result is None, reason="requires invoke package")
 class TestMatchExec(object):
     @patch("paramiko.config.invoke", new=None)
     @patch("paramiko.config.invoke_import_error", new=ImportError("meh"))