Support for custom backends

[tomasfarias]

This PR adds support for custom backends for storing dbt projects and profiles.yml: up until now, only S3 was supported, and local directories worked (kind of) unintentionally.

This PR creates a new abstraction (DbtBackend) to support any backend that implements the necessary push and pull methods. Concrete implementations for the local filesystem and S3 are provided, but in the future more will be added. Backends may use Airflow hooks (DbtS3Backend uses the Amazon provider's S3Hook) but this is not required.

I'm not entirely sure on the naming scheme though: I went with backend since Airflow's XCom also refers as "backend" to the underlying storage and use "push" and "pull" methods for them (see: https://airflow.apache.org/docs/apache-airflow/stable/concepts/xcoms.html#custom-backends). But I'm not super convinced the semantics of the word backend fit what we are doing here as it's much simpler storage. I also thought about:

• "Storage" with "store" and "fetch".
• "Warehouse": discarded as can be confused with something to do with a Data Warehouse.

[codecov]

Codecov Report

Merging #40 (194a975) into master (fc0a12f) will increase coverage by 0.36%.
The diff coverage is 95.48%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
+ Coverage   97.34%   97.70%   +0.36%     
==========================================
  Files           5        8       +3     
  Lines         677      828     +151     
==========================================
+ Hits          659      809     +150     
- Misses         18       19       +1     

Flag	Coverage Δ
unittests	97.70% <95.48%> (+0.36%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
airflow_dbt_python/hooks/backends/__init__.py	86.66% <86.66%> (ø)
airflow_dbt_python/hooks/backends/s3.py	93.10% <93.10%> (ø)
airflow_dbt_python/hooks/dbt.py	97.54% <93.75%> (+1.51%)	⬆️
airflow_dbt_python/operators/dbt.py	99.32% <97.14%> (+<0.01%)	⬆️
airflow_dbt_python/hooks/backends/localfs.py	98.14% <98.14%> (ø)
airflow_dbt_python/hooks/backends/base.py	100.00% <100.00%> (ø)

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[tomasfarias]

Ah, it seems positional-only keywords are not supported in Python 3.7. Even though I don't see any reasons to not upgrade to the latest Python version, I do want to support all the Python versions dbt does.

Edit: I don't see any workaround for this. I'll drop positional-only keywords and eventually merge them once dbt drops Python 3.7.

[tomasfarias]

Python 3.7 support seems to be more tricky than expected...