microscope.device_server.device: default conf to None.

Using a mutable object such as {} as default can lead to surprises
behaviour (see pylint notes for dangerous-default-value / W0102 at
https://pylint.pycqa.org/en/stable/user_guide/messages/warning/dangerous-default-value.html ).
Indeed, this triggered the issues #211, #212, and #274.  So just make
the default None and convert to an empty dict.

Author: David Miguel Susano Pinto

microscope/device_server.py

@@ -81,7 +81,7 @@ def device(
     cls: Callable,
     host: str,
     port: int,
-    conf: Mapping[str, Any] = {},
+    conf: Optional[Mapping[str, Any]] = None,
     uid: Optional[str] = None,
 ):
     """Define devices and where to serve them.
@@ -119,6 +119,9 @@ def construct_devices() -> Dict[str, Device]:
         ]
 
     """
+    if conf is None:
+        conf = {}
+
     if not callable(cls):
         raise TypeError("cls must be a callable")
     elif isinstance(cls, type):