Placeholder package to make the DNF Python API available through PyPI.

A number of DNF's dependencies (notable libdnf and libsolv) are currently only available as part of Linux distro packages.

Right now, this package just reserves the dnf name on PyPI to avoid the potential for a name conflict with the python3-dnf packages on Linux distros using DNF for RPM dependency management.

However, I eventually hope to replace it with a CMake-dependent PEP 517 backend that runs the full libdnf build process, and then emits a statically linked completely self-contained extension module as part of a wheel file (together with the pure Python components of the DNF Python API).