Add SpacyModelArtifact to bentoml.artifacts package

[docteurZ]

Description

Motivation and Context

How Has This Been Tested?

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Documentation
• Test, CI, or build
• None

Components (if applicable)

• BentoService (model packaging, dependency management, handler definition)
• Model Artifact (model serialization, multi-framework support)
• Model Server (mico-batching, logging, metrics, tracing, benchmark, OpenAPI)
• YataiService (model management, deployment automation)
• Documentation

Checklist:

• My code follows the bentoml code style, both ./dev/format.sh and
  ./dev/lint.sh script have passed
  (instructions).
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change requires a change in bentoml/gallery example notebooks
• I have sent a pull request to bentoml/gallery to make that change

[codecov]

Codecov Report

Merging #782 into master will increase coverage by 0.20%.
The diff coverage is 68.32%.

@@            Coverage Diff             @@
##           master     #782      +/-   ##
==========================================
+ Coverage   55.33%   55.53%   +0.20%     
==========================================
  Files         108      108              
  Lines        8158     8144      -14     
==========================================
+ Hits         4514     4523       +9     
+ Misses       3644     3621      -23     

Impacted Files	Coverage Δ
bentoml/saved_bundle/config.py	94.56% <ø> (ø)
bentoml/yatai/deployment/operator.py	61.53% <0.00%> (+8.20%)	⬆️
bentoml/yatai/deployment/sagemaker/wsgi.py	0.00% <0.00%> (ø)
bentoml/yatai/deployment_utils.py	83.72% <ø> (+6.63%)	⬆️
bentoml/yatai/repository/s3_repository.py	25.75% <25.75%> (ø)
bentoml/yatai/client/deployment_api.py	17.30% <33.33%> (ø)
bentoml/yatai/deployment/aws_lambda/operator.py	58.73% <58.73%> (ø)
bentoml/yatai/yatai_service_impl.py	33.83% <75.00%> (ø)
bentoml/yatai/repository/local_repository.py	75.75% <75.75%> (ø)
bentoml/yatai/deployment/sagemaker/operator.py	80.20% <80.20%> (ø)
... and 9 more

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[parano]

Thanks for fixing this @docteurZ

Just a quick note on a workaround for version 0.7.8 and older, user can still access SpacyModelArtifact via from bentoml.artifact.spacy_model_artifact import SpacyModelArtifact

[parano]

Merging now!