Update ics to 0.7.3

[pyup-bot]

This PR updates ics from 0.7 to 0.7.3.

Changelog

0.7.2

*****

This is a bugfix release.

Bug fix:
- Add a lower bound (`>=19.1.0`) on the required version of
`attrs` `353 <https://github.com/ics-py/ics-py/issues/353>`_ (bug introduced in 0.7.1)


*****

0.7.1

*****

This release contains a few minor changes and introduces deprecations for
features that will be removed in 0.8.

Deprecation:
- Add warnings about breaking changes in v0.8 to `Calendar.str()` and `.iter()`.

Minor changes:
- Add a dependency on `attrs <https://pypi.org/project/attrs/>`_.
- Remove the upper bound on the version of `arrow <https://pypi.org/project/arrow/>`_.
- Backport optimizations for TatSu parser from 0.8

Bug fix:
- Fix "falsey" (`bool(x) is False`) alarm trigger (i.e. `timedelta(0)`) not being serialized
`269 <https://github.com/ics-py/ics-py/issues/269>`_

Known bugs:
- Missing lower bound on the required version of `attrs` (`>=19.1.0`) `353 <https://github.com/ics-py/ics-py/issues/353>`_

***********************

Links

• PyPI: https://pypi.org/project/ics
• Changelog: https://data.safetycli.com/changelogs/ics/
• Repo: http://github.com/C4ptainCrunch/ics.py