fix(s3): Split bucket name and key before uploading

[tomasfarias]

S3 files were uploaded to incorrect keys when running Airflow 2. This
was caused by differences between Airflow 1.10 and Airflow 2: the
latter assumes that when passing both bucket_name and key that the key
is to be taken as it is, where as the former seemed to work even when
the key contained the full url.

Now, we do the parsing and splitting ourselves. We would just set
bucket_name to None, however with Airflow 2.0 the upload function
checks for the presence of the bucket_name argument, to decide whether
to parse the url, not if it's set to None (I think this may be a bug).

[tomasfarias]

Closes #46

[tomasfarias]

Will be interesting to see if tests pass as we are now running multiple Airflow versions. If everything's green, then we are good to go.

[codecov]

Codecov Report

Merging #48 (0c9b22c) into master (6c20997) will increase coverage by 0.12%.
The diff coverage is 100.00%.

❗ Current head 0c9b22c differs from pull request most recent head 8ee4bde. Consider uploading reports for the commit 8ee4bde to get more accurate results

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   97.70%   97.82%   +0.12%     
==========================================
  Files           8        8              
  Lines         828      829       +1     
==========================================
+ Hits          809      811       +2     
+ Misses         19       18       -1     

Flag	Coverage Δ
unittests	97.82% <100.00%> (+0.12%)	⬆️

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

Impacted Files	Coverage Δ
airflow_dbt_python/hooks/backends/s3.py	94.31% <100.00%> (+1.21%)	⬆️

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[tomasfarias]

All green. I admit these patches are getting scarier to release, but at some point, I need to start trusting my tests and keep continuously improving them. Bumping patch and shipping