[Housekeeping] Explicitly typing all files!

[akmazian]

I am currently working on explicitly typing all the files and here are my rationales:

• as an end-user, it is sometimes frustrating when I don't know what's type that my functions are gonna return
• as a developer, it makes modifying existing functions and adding new functions much smoother, which can be beneficial in the long run

challenges & solutions / workaround

• backward compatibility: adding types will break the program for end-users using python<3.5. This can be fixed by distributing a separate pyensembl version for python>=3.5. Tools for automatic transpilation of type hints can be utilized in this process

Since it is my first time contributing to a biology-related python package, I am not 100% familiar what the conventions are for such modifications so any feedback would be appreciated!

[iskandr]

I would be happy to take PRs which gradually add types to one (or a small number of related) modules at a time

[akmazian]

I ended up going for a less radical approach lol :) Feel free to provide feedback or change anything!