Added common.sensors package.

The package contains the base types and constants shared across
sensor-based integrations.

platypush/common/sensors.py

@@ -0,0 +1,11 @@
+from typing import Iterable, Mapping, Union
+
+Numeric = Union[float, int]
+SensorDataType = Union[Numeric, Mapping[str, Numeric], Iterable[Numeric]]
+"""
+Numeric sensor data published by integrations can be either of:
+
+    - ``int``/``float``
+    - Mapping of ``str -> int/float``
+    - List of ``int``/``float``
+"""