> for example, the isleap() function which should be defined as is_leap()
Who says that?  Not PEP 8: “Function names should be lowercase, with words separated by underscores *as necessary to improve readability*” (emphasis mine).  isleap is perfectly readable.

> But now it's all too late for that fixes.
You’ll find that Python has a balance between readability/ease of use and pragmatism.  In some cases, changes can’t be made, but it’s best to accept it and move on to the next issue.  Working on feature requests instead of style issues is great.

On a closing note, I’ll let you enjoy the Zen of Python:
$ python -m this