Holdup

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* Fixed buggy handling when http checks are specified with a port.
* Changed User-Agent header and stripped port from Host header for http checks.
* Refactored a bunch of code into a separate ``holdup.checks`` module.

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* Added a static binary in the Github release (built with Pyinstaller on Alpine, as a static bin).
* Dropped support for Python 3.7 and added in Python 3.12 in the test suite.

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* Added support for psycopg 3 (now the ``holdup[pg]`` extra will require that). The old psycopg2 is still supported for now.
* Dropped support for Python 3.6 and added in Python 3.11 in the test suite.

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* Dropped support for Python 2.
* Switched CI from Travis to GitHub Actions.
* Fixed bugs with password masking (it wasn't working for postgresql URIs).

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* Added support for password masking (``--verbose-passwords`` to disable this feature).
* Overhauled checks display a bit, output might be slightly different.
* Added support for basic and digest HTTP authentication.
* Published Docker image at https://hub.docker.com/r/ionelmc/holdup (Alpine based).

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* Added a ``--version`` argument.
* Changed verbose output to mask passwords in postgresql checks.