Replace flake8 with flakeheaven

[weiji14]

Description of proposed changes

So that we can remove the setup.cfg file and have everything in pyproject.toml! Flakeheaven (https://github.com/flakeheaven/flakeheaven) is a flake8 wrapper/fork that is supports configuration in pyproject.toml.

For refererence, these flake8 settings were originally added in #186 and #384.

[flake8]
ignore = E203, E266, E501, W503, F401, E741
max-line-length = 88
#max-complexity = 10
max-doc-length = 79

Addresses #1836.

Reminders

• [ ] Run make format and make check to make sure the code follows the style guide.
• [ ] Add tests for new features or tests that would have caught the bug that you're fixing.
• [ ] Add new public functions/methods/classes to doc/api/index.rst.
• [ ] Write detailed docstrings for all functions/methods.
• [ ] If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• [ ] If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version