Enable flake8

@molinav 已经在做这个了。

开始于 2021年12月29日。

In the context of fixing current issues within `basemap`, one easy step is to use static analysis tools, since they can catch the most evident problems.

I open this issue as a reminder to enable `flake8` in the development workflow; `flake8` will catch the most evident code mistakes, and it will also give advice on changes that will make the code more pep8-compliant. Enabling `pylint` at this moment is not recommended, because it will complain too much.