Enable long-term statistics

[prestomation]

Set state_class to 'measurement' so long-term statistics are generated

Fixes #7

Testing:
I made this on my local copy and noticed this sensor was now an option in the statistics card

[broyuken]

@dreed47 Can you please create a release so that this gets pushed out via a hacs update?

[broyuken]

@prestomation I added in your changes and am getting the following when I reboot HA

Logger: homeassistant.components.sensor
Source: custom_components/redfin/sensor.py:80
Integration: Sensor (documentation, issues)
First occurred: 4:28:11 PM (2 occurrences)
Last logged: 4:28:11 PM

Error adding entities for domain sensor with platform redfin
Error while setting up redfin platform for sensor
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/helpers/entity_platform.py", line 442, in async_add_entities
    await asyncio.gather(*tasks)
  File "/usr/src/homeassistant/homeassistant/helpers/entity_platform.py", line 629, in _async_add_entity
    capabilities=entity.capability_attributes,
  File "/usr/src/homeassistant/homeassistant/components/sensor/__init__.py", line 319, in capability_attributes
    if state_class := self.state_class:
  File "/config/custom_components/redfin/sensor.py", line 80, in state_class
    return STATE_CLASS_MEASUREMENT
NameError: name 'STATE_CLASS_MEASUREMENT' is not defined

any ideas?