Update pre-commit config

[jamesquilty]

After cloning this repository and attempting to commit locally, pre-commit fails at the Initializing environment ... stage. There are at least two problems with .pre-commit-config.yaml:

1. fails on flake8 installation with a request for username and password to login to GitLab because the entry points at https://gitlab.com/pycqa/flake8 which no longer exists;
2. fails to install iSort with error "RuntimeError: The Poetry configuration is invalid"

At this point I gave up and ran pre-commit autoupdate which then also failed to complete environment initialisation because:

3. flake8 0.7.0 (latest) is outside the flake8-comprehensions 3.3.1 version requirements flake8!=3.2.0, <4 and >=3.0 (flake8-comprehensions is specified in the flake8 additional_dependencies section).

Fixing (3) by specifying the latest flake8 3.x version, version 3.9.2, allows pre-commit environment initialisation to complete.

[welcome]

Thanks for opening your first issue here! Engagement like this is essential for open source projects! 🤗

If you haven't done so already, check out EBP's Code of Conduct. Also, please try to follow the issue template as it helps other community members to contribute more effectively.

If your issue is a feature request, others may react to it, to raise its prominence (see Feature Voting).

Welcome to the EBP community! 🎉