mypy and flake8

[eggplants]

I followed flake8 and mypy --strict and fixed their errors. And also I introduced CI of flake8 and mypy.

---

$ git checkout main
$ mypy --strict tests scripts budoux
Found 87 errors in 11 files (checked 15 source files)
$ flake8 | wc -l
331

$ git checkout mypy
$ mypy --strict tests scripts budoux
Success: no issues found in 14 source files
$ flake8 | wc -l
0
$ grep -r 'type: ignore' | grep -v .git
tests/test_train.py:from scripts import train  # type: ignore # noqa (module hack)
$ grep -r noqa | grep -v .git
tests/test_parser.py:from budoux import parser  # noqa (module hack)
tests/test_feature_extractor.py:from budoux import feature_extractor, utils  # noqa (module hack)
tests/test_train.py:from scripts import train  # type: ignore # noqa (module hack)
tests/test_main.py:from budoux import main  # noqa (module hack)
scripts/context.py:from budoux import Parser, feature_extractor, utils  # noqa (unused)
scripts/train.py:           == True) - 1  # noqa (cannot replace `==` with `is`)

[eggplants]

Type subscription in numpy (like np.floating[typing.Any]) is available on Python >=3.9.
https://github.com/numpy/numpy/blob/bb95cf015165bcf85f99ba0d28a7a6e727ca955b/numpy/core/src/multiarray/scalartypes.c.src#L1843-L1868
It is avoidable with replacement to inline type comment.

[tushuhei]

Thank you for a lot of improvements to make our code typed! Let me ask some minor fixes.

[tushuhei]

LGTM. Thank you so much!