Fiona: boto3 isn't optional

[sgillies]

boto3 is intended to be optional, but is not. I uninstalled boto3 from my dev environment and get this failure when running fio-info on a local shapefile.

$ fio info tests/data/coutwildrnp.shp
ERROR:fio:Exception caught during processing
Traceback (most recent call last):
  File "/Users/seang/code/Fiona/fiona/fio/info.py", line 50, in info
    with fiona.open(input, layer=layer) as src:
  File "/Users/seang/code/Fiona/fiona/__init__.py", line 176, in open
    enabled_drivers=enabled_drivers, **kwargs)
  File "/Users/seang/code/Fiona/fiona/collection.py", line 144, in __init__
    self.env = AWSGDALEnv()
  File "/Users/seang/code/Fiona/fiona/drvsupport.py", line 159, in __init__
    import boto3
ImportError: No module named boto3
Aborted!

Copying over more of the implementation from rasterio.env will be the way to go.

[schwehr]

And even if boto3 is present, not everyone wants it to go off and talk to AWS to run the default tests. I am currently working on how to have testing in a locked down env.

[sgillies]

@schwehr my recent work in rasterio is going to improve the situation quite a bit once it's ported back here.

[sgillies]

Having refactored Rasterio's cloud connectivity, I'm getting started on doing so here.