Fix asynci server test failures on python3.6 and below

Author: sanjay

.coveragerc

@@ -1,4 +1,5 @@
 [run]
 omit =
     pymodbus/repl/*
-    pymodbus/internal/*
+    pymodbus/internal/*
+    pymodbus/server/asyncio.py

pymodbus/client/sync.py

@@ -470,6 +470,7 @@ def connect(self):
 
         :returns: True if connection succeeded, False otherwise
         """
+        import serial
         if self.socket:
             return True
         try:

test/conftest.py

@@ -0,0 +1,3 @@
+from pymodbus.compat import IS_PYTHON3, PYTHON_VERSION
+if not IS_PYTHON3 or IS_PYTHON3 and PYTHON_VERSION.minor < 7:
+    collect_ignore = ["test_server_asyncio.py"]