Add typestub

[tamuhey]

Based on PEP 561, I added type stubs into mojimoji.

The changes are as follows:

• add mojimoji-stubs directory which contains type stub file (ref: https://www.python.org/dev/peps/pep-0561/#stub-only-packages)
• add mypy to test the type stub
• refactor test

[ikuyamada]

@tamuhey Thank you for your PR! Do you also plan to distribute mojimoji-stub package on PyPI?

[tamuhey]

@ikuyamada Of course!
But according to PEP 561, mojimoji-stubs is automatically bundled in mojimoji when it is packaged, so there is no need to publish mojimoij-stubs separately.

PEP 561:

... the stubs can also be put in a separate package and distributed separately. Third parties can also find this method useful if they wish to distribute stub files. The name of the stub package MUST follow the scheme foopkg-stubs for type stubs for the package named foopkg. Note that for stub-only packages adding a py.typed marker is not needed since the name *-stubs is enough to indicate it is a source of typing information.

[tamuhey]

So I want to release next version of mojimoji, what do you think? @ikuyamada

[ikuyamada]

@tamuhey Thank you for the information and the contribution! It would be helpful for users to bundle the stub file, so I agree to release the next version of mojimoji.

[tamuhey]

I tried it with test pypi but it didn't work.
I'm sorry I misunderstood somthing.
I'll fix it in another PR.