clean out the last_* fields of sys before importing it.

The system gets confused if there were uncaught errors in previous
tests without this. Particularly, it crashes (at least 2.6) if any tests during
test_integrations were skipped.

test/test_api/test_api.py

@@ -9,6 +9,13 @@ from jedi._compatibility import is_py3
 from pytest import raises
 
 
+def _clear_last_exception_fields_in_sys():
+    import sys
+    sys.last_type = None
+    sys.last_value = None
+    sys.last_traceback = None
+
+
 def test_preload_modules():
     def check_loaded(*modules):
         # +1 for None module (currently used)
@@ -20,6 +27,7 @@ def test_preload_modules():
     temp_cache, cache.parser_cache = cache.parser_cache, {}
     parser_cache = cache.parser_cache
 
+    _clear_last_exception_fields_in_sys()
     api.preload_module('sys')
     check_loaded()  # compiled (c_builtin) modules shouldn't be in the cache.
     api.preload_module('json', 'token')