Add py.typed

[juanjosegarciaripoll]

Could you consider adding py.typed to the module directory, and installing this file, so that mypy does not complain when importing opt_einsum?

[dgasmith]

As commented in #246 as well, can you try manually adding a py.typed to an install and see if you have any issues with the typing? I haven't been able to verify the typing doesn't cause additional issues due to the duck typed nature of Arrays in the library.