feat: use jinja for templating

[skellet0r]

What I did

Redid the mock ERC20 contract and used Jinja2 templating to clean/simplify the process of generating contracts with different return values / failure conditions.

Previously, we did manual string manipulation, however that is very clunky and doesn't invite modification. Using jinja however, the source modifications can be done in the contract file itself, rather than via the python format function.

This also serves as an example for future modifications / templates. Although unused by me, there could very well be ERC721, ERC777, ERC1363, ERC1155 ... and more token templates.

How I did it

Mainly using the Jinja2 documentation https://jinja.palletsprojects.com/, the templating syntax is rather simple to catch on to, and has a rich set of features (such as macros which enable DRY code).

How to verify it

I ported over an actual set of ERC20 test cases, to verify that the contract works properly (not just return values), The tests are parametrized over the set of possible variations:

Success Value: True, False, None
Failure Value: True, False, None, "revert"

Checklist

• I have confirmed that my PR passes all linting checks
• I have included test cases
• I have updated the documentation (README.md)
• I have added an entry to the changelog