Add requirements.in for users using pip-compile

[ajfriend]

Some projects use pip-compile to generate a requirements.txt file, but h3-py uses the standardized pyproject.toml.
There seems to be some interest to make pip-compile compatible with pyproject.toml: jazzband/pip-tools#1047

In the meantime, we could consider adding a requirements.in file like:

# Required for pip-compile until
# https://github.com/jazzband/pip-tools/issues/1047 is resolved.
#
# In the meantime, this information is duplicated from
# pyproject.toml

scikit-build
cython
cmake

[ajfriend]

Closed by #157