Flit-core backend for pure python builds

[tammoippen]

fixes #70

This pull request updates the packaging and build configuration for the plotille project, primarily switching the build backend from uv_build to flit_core and incrementing the project version. These changes help modernize the build process and prepare for future releases.

Packaging and build system updates:

• Updated project version from 6.0.4 to 6.0.5 in pyproject.toml to reflect new changes.
• Changed the build backend from uv_build to flit_core and updated the requires field to use flit_core instead of uv_build in pyproject.toml.

Other configuration changes:

• Removed the License :: OSI Approved :: MIT License classifier from the classifiers list in pyproject.toml - requirement from flit-core.