Finish PythonCall.jl integration

The vast majority of the work to integrate PythonCall.jl has been done here https://github.com/avik-pal/Wandb.jl/pull/22.

Here are the changes that I make in this PR:

Use conda to install all dependencies to avoid version issues with having pip and conda deps.
Limit protobuf to v3 (v4 appears to have compatibility issues, and I couldn't import wandb with it).
Add FluxTraining.jl example
Fix Flux.jl, mpi examples
Fix mime_pairs logic in Wandb.Image
Fix the test by converting the pystring to a julia string

I did not add any extra tests, but all the examples run for me.

avik-pal / Wandb.jl