fix: move openexr to runtime dependency

[jonas-eschle]

OpenEXR is meant to be an optional dependency as raised in https://github.com/tensorflow/graphics/issues/421.

Moved it away from the requirements.txt (where it probably never should have been) and added a runtime check for the import and version.

Alternatively, having extras like [dev] or [openexr] in the requirements could be a step forward, in general, to make this better manageable.

[jonas-eschle]

@nkovela1 can we maybe get this merged? If it needs to be, I can also become a maintainer of the repo to help with PRs

This is actively preventing people from installing the package, unfortunately

[nkovela1]

I will defer to @G4G on this, as he is the owner of the TF graphics repo.