Avoid SPIDevice init failure in absence of libgpiod bindings

[ilario]

When importing other Adafruit components, I observed this error:

[...]
    from adafruit_register.i2c_struct import ROUnaryStruct, UnaryStruct
  File "/opt/measurement_system-venv/lib/python3.9/site-packages/adafruit_register/i2c_struct.py", line 22, in <module>
    from circuitpython_typing.device_drivers import I2CDeviceDriver
  File "/opt/measurement_system-venv/lib/python3.9/site-packages/circuitpython_typing/device_drivers.py", line 13, in <module>
    from adafruit_bus_device.spi_device import SPIDevice
  File "/opt/measurement_system-venv/lib/python3.9/site-packages/adafruit_bus_device/spi_device.py", line 29, in <module>
    class SPIDevice:
  File "/opt/measurement_system-venv/lib/python3.9/site-packages/adafruit_bus_device/spi_device.py", line 76, in SPIDevice
    chip_select: Optional[DigitalInOut] = None,
NameError: name 'DigitalInOut' is not defined

This is happening because I did not have the "libgpiod Python bindings" (installable with pip install gpiod) and the import here fails:

Adafruit_CircuitPython_BusDevice/adafruit_bus_device/spi_device.py

Line 20 in 519bf36

from digitalio import DigitalInOut

Causing this line to fail:

Adafruit_CircuitPython_BusDevice/adafruit_bus_device/spi_device.py

Line 76 in 519bf36

chip_select: Optional[DigitalInOut] = None,

This started happening with adafruit/Adafruit_CircuitPython_Typing#42 as it introduced importing adafruit_bus_device.spi_device.SPIDevice.