Zopfli

Dropped support for Python 3.7, added support for Python 3.12. No other code changes.

* Added pyproject.toml to source distribution
* Published precompiled wheels for Python 3.11

- [zopfli.png] Only print debug info to console output when -v/--verbose flag is used.

- Dropped support for EOL Python 2.7 and 3.6, require Python 3.7+ (16).
- Added support for compressing PNGs via `zopfli.png.optimize` method. Also available from the command line as `python -m zopfli.png`; mimics the upstream's `zopflipng` c++ executable (17).

- Allow to link against system installed zopfli library by setting USE_SYSTEM_ZOPFLI environment variable (9).
- Added wheels for Python 3.10, universal2 macos "fat" (x86_64 + arm64) architecture, Linux aarch64 and pypy3 (10, 11).
- Determine package version automatically from latest git tag using setuptools_scm.

Fixed automatic upload from Travis to PyPI.