Move `How not to use is operator` snippet code from README to Python file

For now there is no clear description in `CONTRIBUTING.md`, so please use existing example as a reference
- [snippet in README](https://github.com/satwikkansal/wtfpython/blob/dev/README.md#L289-L356)
- [Python file](https://github.com/satwikkansal/wtfpython/blob/dev/snippets/2_tricky_strings.py)
- [add line in noxfile](https://github.com/satwikkansal/wtfpython/blob/dev/noxfile.py#L13)