CI clean pre-commit: upgrade isort, use correct file types in flake8

[MarcoGorelli]

A few things here:

• updating isort version

• updating to use types instead of files (see Matching cython files: merge with python or support separately? pre-commit/pre-commit#1436 (comment) and Let pre-commit sort pxd files too PyCQA/isort#1549 (comment))

• removing exclude: ^pandas/__init__\.py$|^pandas/core/api\.py$ as it's no longer necessary

• replacing files: \.(pyx|pxd)$ with types: [cython]:

  $ 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)

[jbrockmendel]

Nice! Great work with the CI/code_check improvements

[jreback]

thanks @MarcoGorelli

[MarcoGorelli]

Nice! Great work with the CI/code_check improvements

Thanks, much appreciated