Pyright reports false positive error with asynccontextmanager (#87)

Pyright gets confused and will give you error messages like this:
```
error: Object of type "AsyncIterator[AsyncGenerator[...]]" cannot be used with "with" because it does not implement __aenter__ (reportGeneralTypeIssues)
error: Object of type "AsyncIterator[AsyncGenerator[...]]" cannot be used with "with" because it does not implement __aexit__ (reportGeneralTypeIssues)
```
For code like this:
```python
async with Client("test.mosquitto.org") as client:
    async with client.filtered_messages("floors/+/humidity") as messages:
        await client.subscribe("floors/#")
        async for message in messages:
            print(message.payload.decode())
```

This change statically checks against `sys.version_info` for the right version, which Pyright understands.

asyncio_mqtt/client.py

@@ -3,6 +3,7 @@
 import logging
 import socket
 import ssl
+import sys
 from contextlib import contextmanager, suppress
 from enum import IntEnum
 from types import TracebackType
@@ -27,9 +28,9 @@
 
 
 
-try:
+if sys.version_info >= (3, 7)
     from contextlib import asynccontextmanager
-except ImportError:
+else:
     from async_generator import asynccontextmanager as _asynccontextmanager # type: ignore
     from typing_extensions import ParamSpec
     _P = ParamSpec('_P')