Remove pre 7.0 compatibility; other cleanup

[dhalbert]

Simplify and shorten the current code. I will have a further PR later that changes a bunch of thrown exceptions to allow for easier error handling.

• CircuitPython 7 supports split() on bytes and bytearrays. Remove hand-coded routines no longer needed.
• CircuitPython 6 supports streaming json decoding. Remove workaround for 5.x non-streaming decoding.
• Refactor the duplicated check for gzip content.

Please test with your own wifi code - thanks. I tested this with the simple wifi testing code, and with the Google calendar example, but not with other examples.

[dhalbert]

Re-pushed; due to git problems, the code that was pushed was wrong.

[anecdata]

Passes initial testing with my "production" code on PyPortal and Metro ESP32-S2 (CircuitPython 7.2.0). I'll load it on more devices and let it run.

[askpatrickw]

I ran pytest locally and hit the same failure as the CI.

Changing https://github.com/adafruit/Adafruit_CircuitPython_Requests/blob/main/tests/post_test.py#L83 to sock.send.assert_called_with(b'Date=July 25, 2019')

Fixes the test... well it passed. But I don't understand why. ;-)

[dhalbert]

Also tested with a PyPortal (ESP32 coprocessor) with the OpenWeather demo. Ready for re-review.