CircuitPython API Compatibility and Bugfixes

[brentru]

This pull request:

• Removes methods which rely on a native call to time.strftime.

  • CircuitPython's time module does not implement strftime
  • ctime() added to datetime so users can still do string-format-y things.

• Fix an OverflowError within timestamps on ESP32-S2

• gmtime() not implemented within CircuitPython time API (add gmtime() to time module circuitpython#1663) - raise error where appropriate.

• Examples and tests modified to reflect the changes above.

• Fix broken tzinfo.__new__ call.

• Tested on Adafruit CircuitPython 6.1.0-beta.2 on 2020-12-03; MagTag with ESP32S2

[askpatrickw]

I used this PR in a project and it worked.

offset_adjusted_dt = datetime.fromtimestamp(timestamp) - timedelta(hours=abs(offset_hours))
            return offset_adjusted_dt.isoformat()

I ran the examples and they worked.
I ran the tests and there were 44 skips. If that's expected, it looks good to me.

python -m unittest test_datetime.py 
..s.ss...s..ssss..s.s...ss..s.sssssssssss...sssss..ss..ssssssss..s.s..s.ss.
----------------------------------------------------------------------
Ran 75 tests in 0.014s

OK (skipped=44)

[brentru]

@askpatrickw Thanks! Could you mark this as "Approved" ?

[askpatrickw]

@brentru Are the skips all valid and OK?

[brentru]

@askpatrickw Yep. This library implements a subset of datetime, and some portions of datetime aren't possible yet (i.e: CircuitPython time API doesn't support gmtime, strftime)

[askpatrickw]

Skips explained by Brent. Thanks!