Ecsjobs

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* Bump to 1.0.0 since I've been using this for years
* Build and test against Python 3.9; build Docker image off of Python 3.9
* Add ``rsnapshot`` apk to Docker image
* Updates for new pytest version

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* Fix deprecated PyYAML load calls.

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* Relax PyYAML version in order to work with Python 3.9.

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* Add leading newlines when passing report to ``failure_command``
* In the ``failure_html_path`` global config setting, the string ``{date}`` will be replaced with the current datetime (at time of config load) in ``%Y-%m-%dT%H-%M-%S`` format.

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* Add ``awscli`` to Docker image
* Add new global configuration options:

* **failure_html_path** - *(optional)* a string absolute path to write the HTML email report to on disk, if sending via SES fails. If not specified, a temporary file will be used (via Python's ``tempfile.mkstemp``) and its path included in the output.
* **failure_command** - *(optional)* Array. A command to call if sending via SES fails. This should be an array beginning with the absolute path to the executable, suitable for passing to Python's ``subprocess.Popen()``. The content of the HTML report will be passed to the process on STDIN.

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* Document release process
* Document how to run in ECS as a Scheduled Task
* ``LocalCommand`` - if ``script_source`` parameter is specified, instead of ignoring ``command`` send it as arguments to the downloaded script.
* ``LocalCommand`` bugfix - Handle when retrieved script_source is bytes instead of string.
* Add ``-m`` / ``--only-email-if-problems`` command line argument to allow suppressing email reports if all jobs ran successfully.
* Make report email subject configurable via ``email_subject`` global configuration option.