Fix/requirements

[justindujardin]

Refactor Pathy to support conditionally installing packages for cloud providers. Similar to smart_open we can expect to have the following package extras:

pip install pathy # no dependencies, only fs adapter will work
pip install pathy[gcs] # can work with gs://foo/bar/paths
pip install pathy[s3] # can work with s3://foo/bar/paths (eventually, not implemented)

Changes:

• [x] Add "gcs" extras to package
• [x] Tests pass without gcs packages installed
• [x] Simplify some BucketClient types (e.g. ClientBucket -> Bucket, ClientBlob -> Blob)
• [x] Better StreamableType for smart_open streamable objects
• [x] Update codebase to support looking up BucketClients by their scheme

[codecov[bot]]

Codecov Report

Merging #30 into master will increase coverage by 0.84%. The diff coverage is 81.56%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   78.37%   79.21%   +0.84%     
==========================================
  Files           8        9       +1     
  Lines         823      871      +48     
==========================================
+ Hits          645      690      +45     
- Misses        178      181       +3     

Impacted Files	Coverage Δ
pathy/gcs.py	28.35% <34.88%> (-2.20%)	:arrow_down:
pathy/client.py	88.88% <84.61%> (+20.07%)	:arrow_up:
pathy/api.py	89.47% <92.59%> (-0.99%)	:arrow_down:
pathy/clients.py	93.10% <93.10%> (ø)
pathy/__init__.py	100.00% <100.00%> (ø)
pathy/base.py	96.82% <100.00%> (+3.49%)	:arrow_up:
pathy/cli.py	90.90% <100.00%> (ø)
pathy/file.py	83.55% <100.00%> (+0.33%)	:arrow_up:

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[justindujardin]

:tada: This PR is included in version 0.3.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: