setup.py fails with error "sh: cython: command not found"

(Even with `cython` installed.) To reproduce:

```
% pyvenv-3.3 env3
# install pip.
% env3/bin/pip install cython 2>&1 >/dev/null
% ls -l env3/bin/cython
-rwxr-xr-x  1 me  staff  316 May  5 13:21 env3/bin/cython
% env3/bin/pip install git+https://github.com/surfly/gevent.git#egg=gevent-1.0.1dev
Downloading/unpacking gevent-1.0.1dev from git+https://github.com/surfly/gevent.git
  Cloning https://github.com/surfly/gevent.git to ./env3/build/gevent-1.0.1dev
  Running setup.py (path:/Users/royiv/tmp/env3/build/gevent-1.0.1dev/setup.py) egg_info for package gevent-1.0.1dev
    /Users/royiv/tmp/env3/bin/python util/cythonpp.py -o gevent.corecext.c gevent/core.ppyx
    sh: cython: command not found
    Running cython -o gevent.corecext.c gevent/core.pyx  # !EV_USE_SIGNALFD && !defined(LIBEV_EMBED) && !defined(_WIN32)
    Traceback (most recent call last):
      File "util/cythonpp.py", line 772, in <module>
        process_filename(filename, options.output_file)
      File "util/cythonpp.py", line 87, in process_filename
        output = run_cython(pyx_filename, sourcehash, output_filename, banner, comment)
      File "util/cythonpp.py", line 535, in run_cython
        system(command, comment)
      File "util/cythonpp.py", line 545, in system
        raise AssertionError('%r failed with code %s' % (command, result))
    AssertionError: 'cython -o gevent.corecext.c gevent/core.pyx' failed with code 32512
    make: *** [gevent/gevent.corecext.c] Error 1
    Running 'make' in /Users/royiv/tmp/env3/build/gevent-1.0.1dev
    Complete output from command python setup.py egg_info:
    /Users/royiv/tmp/env3/bin/python util/cythonpp.py -o gevent.corecext.c gevent/core.ppyx

sh: cython: command not found

Running cython -o gevent.corecext.c gevent/core.pyx  # !EV_USE_SIGNALFD && !defined(LIBEV_EMBED) && !defined(_WIN32)

Traceback (most recent call last):

  File "util/cythonpp.py", line 772, in <module>

    process_filename(filename, options.output_file)

  File "util/cythonpp.py", line 87, in process_filename

    output = run_cython(pyx_filename, sourcehash, output_filename, banner, comment)

  File "util/cythonpp.py", line 535, in run_cython

    system(command, comment)

  File "util/cythonpp.py", line 545, in system

    raise AssertionError('%r failed with code %s' % (command, result))

AssertionError: 'cython -o gevent.corecext.c gevent/core.pyx' failed with code 32512

make: *** [gevent/gevent.corecext.c] Error 1

Running 'make' in /Users/royiv/tmp/env3/build/gevent-1.0.1dev

----------------------------------------
Cleaning up...
Command python setup.py egg_info failed with error code 1 in /Users/royiv/tmp/env3/build/gevent-1.0.1dev
Storing debug log for failure in /Users/royiv/.pip/pip.log
```