prior: fix AttributeError for bad connections

When constructing a ProScanIII object on a port that does
not contain a prior controller, the _devices member is not
defined.

This causes the __del__ operation of the object to fail, because
it will end up in the base class' __del__, which requires the
`devices` property, which requires the `_devices` member to be present.

Conceptually, the `abc.Controller` class requires the implementation to be
a valid object.  the `super().__init__` method may call any of the
public base members, so we have to make sure they are valid before
calling it.

One may also argue that the object is not allowed
to be constructed at all if not connected to a proper device; that
would be an alternative fix.

microscope/controllers/prior.py

@@ -211,16 +211,18 @@ class ProScanIII(microscope.abc.Controller):
     def __init__(
         self, port: str, baudrate: int = 9600, timeout: float = 0.5, **kwargs
     ) -> None:
-        super().__init__(**kwargs)
-        self._conn = _ProScanIIIConnection(port, baudrate, timeout)
         self._devices: Mapping[str, microscope.abc.Device] = {}
 
+        self._conn = _ProScanIIIConnection(port, baudrate, timeout)
+
         # Can have up to three filter wheels, numbered 1 to 3.
         for number in range(1, 4):
             if self._conn.has_filterwheel(number):
                 key = "filter %d" % number
                 self._devices[key] = _ProScanIIIFilterWheel(self._conn, number)
 
+        super().__init__(**kwargs)
+
     @property
     def devices(self) -> Mapping[str, microscope.abc.Device]:
         return self._devices