Make dependencies on boto3 optional

[jhnnsrs]

Hey everyone,

first of all thanks for this amazing library, and all of the efforts of bridging the two ecosystems!
I noticed that boto3 is listed as a dependency with python-bioformats. As it is not necessary for the core of the library i was wondering if you would consider making this dependency optional and factoring out the logic to connect to s3 and omero?

I am also happy to provide a PR !