add py.typed file for PEP-561 compatibility

[helpmefindaname]

PEP-561 is the standard for adding typehints to packages.
As flair itself has already type hints for most methods, simply adding a py.typed file to the package data.

To explain further of why PEP-561 is useful:

let's create an environment installing flair==0.11.3 and mypy.
And create a file example_flair.py with the following content:

from flair.data import Sentence


data = Sentence("hello world")

data[0] = "abc"

Note that this is clearly a violation as Sentence doesn't implement __setitem__ and data[0] would be rather a Token instead of a str

running mypy example_flair.py lead to the following error:

example_flair.py:1: error: Skipping analyzing "flair.data": module is installed, but missing library stubs or py.typed marker
example_flair.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

A user of flair can handle this by adding type: ignore to the import, or add the following to the mypy config:

[mypy]
ignore_missing_imports = True

either way running mypy example_flair.py would pass by ignoring all potential errors:
Success: no issues found in 1 source file

However, if we install flair locally, we get the errors we would expect from our incorrect script:

example_flair.py:6: error: Unsupported target for indexed assignment ("Sentence")
Found 1 error in 1 file (checked 1 source file)

Simply having this py.typed file within the package data is enough to provide the users the full mypy warnings for any missusages.

(Note that we have set include_package_data=True, therefore the py.typed file will be automatically added to the package data without further describing it.

[alanakbik]

Thanks for adding this and providing the explanation!