Verify drivers correctly import `struct`

[tannewt]

In 2.x struct is available as ustruct. We need to make sure all drivers included in the bundle will work with 3.x which has renamed it to struct.

For backwards compatiblity it should be:

try:
    import struct
except ImportError:
    import ustruct as struct

[jepler]

I've grepped around in the circuitpython master branch source tree, its submodules, and Adafruit_CircuitPython_Bundle and its submodules and I didn't find any further problems. There are at least a few which do the reverse of what is suggested, though, such as ports/esp8266/modules/ntptime.py:

try:
    import ustruct as struct
except:
    import struct

@tannewt is there somewhere else I should look, or should I go ahead and close this issue up?

[tannewt]

The reverse is fine in ESP8266 modules. I'll close now. Thanks!