Adding Arbitrum Mainnet DAI

Adding py3.9

Adding pip check to the CI pipeline

Removing legacy resolver as the issue was resolved by fixing dependencies

Reverting dependabot updates to speed up pip install with the new resolver

Adding `/.idea` to .gitignore to avoid commiting pycharm's internal rubbish by accident

Pinning Pip to 21.0.0 for tox to run with legacy resolver, didn't find an option to enable legacy resolver for now

Removing Python3.6 from tox, adding Py3.9 to tox.ini

temporarily disabling Py3.6 to see if CI passes

Trying to add wheel and setuptools to resolve installation issues, they are used by pretix, so I'm assuming we can add them to the plugin requirements risk-free

Added --use-deprecated=legacy-resolver option to pip install until the timeout issue is resolved in pip or in any other way

Update urllib3 requirement from <1.26.0 to <1.27.0

Update eth-utils requirement from <2,>=1.8.4 to >=1.8.4,<3

Update eth-abi requirement from <3,>=2.1.1 to >=2.1.1,<4

Update flake8 requirement from <4,>=3.7 to >=3.7,<5