Allow pathlib.Path for template_folder

[ivanovmg]

This PR allows template_folder argument to be a pathlib.Path object.

• fixes Allow template_folder to be a pathlib.Path object #4892

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

I added a new test, which ensures that template_folder can be pathlib.Path object.
However, when I added this tests, it was already successful.
I noticed that there are no similar tests of this kind in regards to template_folder.

I suppose that the issue referenced is related to typing only, because according to the successful test, one can use this out of the box, and there is no need to convert it into a str object.

app = Flask(
    __name__,
    template_folder=pathlib.Path("templates"),
)

[legoktm]

Thank you!