Circuitpython-csv

1. Updates missing and incorrect type annotations
2. Adds typing to Sphinx RTD documentation

- Corrected RTD documentation formatting in docstrings
- Finalized and corrected type hints
- Using format() for string formatting
- writeheader() doesn't return anything now, to keep consistency with the rest of the library; this may be changed in the future

writerow method will now convert all elements to strings

Adds basic csv reading and writing functionality, similar to CPython's csv module, including

- csv.reader
- csv.writer
- csv.DictReader
- csv.DictWriter

To use in CircuitPython, download the .mpy file and copy it to the `lib` folder on the `CIRCUITPY` drive. Or, simply install the [Community bundle](https://github.com/adafruit/CircuitPython_Community_Bundle).

Read the [docs](https://circuitpython-csv.readthedocs.io/en/latest/) for info on how to use it.