CI clean pre-commit: upgrade isort, use correct file types in flake8
A few things here:

- updating isort version
- updating to use `types` instead of `files` (see https://github.com/pre-commit/pre-commit/issues/1436#issuecomment-624219819 and https://github.com/PyCQA/isort/pull/1549#issuecomment-706621474)
- removing `exclude: ^pandas/__init__\.py$|^pandas/core/api\.py$` as it's no longer necessary
- replacing `files: \.(pyx|pxd)$` with `types: [cython]`:

  ```bash
  $ identify-cli pandas/_libs/groupby.pyx
  ["cython", "file", "non-executable", "text"]
  $ identify-cli pandas/_libs/lib.pxd
  ["cython", "file", "non-executable", "text"]
  ```

- replacing the name `flake8-pxd` with `flake8 (cython template)`, as that hook wasn't actually checking `.pxd` files (the `flake8-pyx` one was)