Fixing flake8 warnings and errors

flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics --exclude benchmark

Excluding the benchmark directory (due to protobuf generated files)
Also removing some Python2 specific code

examples/async_ssl_server.py

@@ -78,7 +78,7 @@ async def main():
             addr, port,
             ssl=ctx,
         )
-    except:
+    except Exception:
         print("Try IPv6")
         server = await asyncio.start_server(
             myserver,