Remove __future__ annotations from Python files

[sobolevnrm]

Note that __future__ annotations is not required anymore in all files.

Originally posted by @kiyoon in #414 (comment)

[sobolevnrm]

@kiyoon do you want me to address this in a new PR or should it be added to your existing PR #412 ?

[kiyoon]

I can update that

[sobolevnrm]

@kiyoon are any of the following lines still needed?

• pdb2pqr/pdb2pqr/residue.py

  Line 7 in 5ebbae4

  from __future__ import annotations

• pdb2pqr/pdb2pqr/main.py

  Line 10 in 5ebbae4

  from __future__ import annotations

[kiyoon]

@sobolevnrm That's already removed just now, right?

[sobolevnrm]

You're right: I checked the wrong version of the master branch.