Remove `requirements.txt` to avoid confusion

By convention, `requirements.txt` is used to describe concrete Python
environments, but in this project the file was used to describe package
dependencies. Thus, remove `requirements.txt` and declare dependencies
in the `setup.py` script directly.

MANIFEST.in

 include LICENSE
 include README.md
-include requirements.txt

requirements.txt

-Jinja2
-Pillow