add missing requirements-pypy3.txt

After ce7b8dbf42de27eddd6e76b096c57d7f1abe845c all travis-ci jobs are
failing the pypy3 tests with `Could not open requirements file: [Errno 2] No such file or directory: 'requirements-pypy3.txt'`
Merge pull request #223 from dnozay/pypy3

add missing requirements-pypy3.txt