Repro-zipfile

- Adds support for type checking per [PEP 561 specification](https://typing.python.org/en/latest/spec/distributing.html#packaging-typed-libraries). This is implemented through a `.pyi` stubs file. ([Issue 12](https://github.com/drivendataorg/repro-zipfile/issues/12), [PR #17](https://github.com/drivendataorg/repro-zipfile/pull/17))
- Changed `date_time` function to always return the date-time value as 6-tuple of `int` values.

- Fixed bug that caused timestamps set by `SOURCE_DATE_EPOCH` to be affected by the local system timezone. It now always uses UTC. ([PR 8](https://github.com/drivendataorg/repro-zipfile/pull/8) from [thatch](https://github.com/thatch))

- Added a `cli` installation extra for installing the rpzip package, which includes a command-line program

- Changed `ReproducibleZipFile` to also overwrite file-system permissions with fixed values. These default to `0o644` (`rw-r--r--`) for files and `0o755` (`rwxr-xr-x`) for directories.
- Added support for `REPRO_ZIPFILE_FILE_MODE` and `REPRO_ZIPFILE_DIR_MODE` environment variables for overriding the fixed file and directory permission values.

Initial release! 🎉