Fix bug in fsspec exists call

See merge request repos/data-engineering/workflow_utils!8

Add a Gitlab CI pipeline.

See merge request repos/data-engineering/workflow_utils!6

This reverts commit 07a5035d.

Adds a Gitlab CI pipeline to run tests, mypy
and linting on python 3.7 and 3.9.

Automate usage of fsspec hdfs URLs via new pyarrow HDFS API

See merge request repos/data-engineering/workflow_utils!5

- fsspec_use_new_pyarrow_api - call this to make fsspec always use
  new pyarrow API with all hdfs:// URLs.
  This is only needed until
  https://github.com/fsspec/filesystem_spec/issues/874 is resolved.

- set_hadoop_env_vars - sets needed env vars to work with new pyarrow HDFS API.
  This is also called by fsspec_use_new_pyarrow_api() by default.

https://phabricator.wikimedia.org/T300876

There seems to be an issue with airflow and pyarrow, at least
in SequentialExecutor, where a call to pyarrow in the scheduler
can cause deadlock.  However, when determining the cached
url for an artifact, we'd like to be able to reference it
without checking that it exists (which in HDFS would result
in a call through pyarrow).  This avoids that call if we
don't care if the artifact exists at this stage.

There is a syntax bug in an older version

Regen docs

Also rebuild sphinx docs

Conflicts:
	README.md
	pytest.ini
	setup.cfg