cython: drop egg-info; fix python shebang

- Drop egg-info, not needed
- Use python via /usr/bin/env, otherwise it hardcodes not-relocated
  full path to Python. This generates unwanted RPM dependencies, e.g.,

  rpm -qp --requires external+cython+0.19.1-1-1.fc19_aarch64_gcc490.rpm
  [snip]
  /home/david.abdurachmanov/real-arch/allext/a/fc19_aarch64_gcc490/external/python/2.7.6/bin/python
  [snip]

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #870 from davidlt/cython-python-dep-fix

cython: drop egg-info; fix python shebang