Add support for "fake" MCP2221

[brentru]

This pull request adds Blinka support for a "fake" MCP2221 device. This means that no device is attached and the environment will mock the CircuitPython hardware API layer.

To test, set the following environment variables in either your shell or a CPython script:

os.environ["BLINKA_FORCEBOARD"] = "MICROCHIP_MCP2221"
os.environ["BLINKA_ FORCECHIP"] = "MCP2221"

Implemented and tested on CPython:

• Digital Input
  • Returns a True/False value, toggles between them on each call
• Analog Input
  • Returns an arbitrary 16-bit value
• I2C
  • Scan (returns reasonable values, otherwise can feed the scan() function it a list of arguments as a test stimuli)
  • Support for "Fake BME280" driver
    • This driver mocks the functionality of the Adafruit_CircuitPython_BME280, allowing you to test your code without attaching physical hardware. Instead of using random data, weather data is instead pulled from the OpenWeatherMaps API and returned as if it were coming from the BME280 sensor properties.

[makermelissa]

There are a bunch of code blocks where it leaves the slimmest possibility of it not functioning as intended. Other than that, it looks good.

[makermelissa]

Looks great. Thank you.