[#248] Add Semaphore._py3k_acquire for PyPy 2.4

This fixes some tests which use the Semaphore module with PyPy 2.4.0. See https://github.com/surfly/gevent/issues/248#issuecomment-59434565 for explanation

After patch on Ubuntu:

```
2/123 unexpected passes
 - /home/vagrant/pypy/bin/python -u test__exc_info.py
 - /home/vagrant/pypy/bin/python -u test__real_greenlet.py

19/123 tests failed in 02:25

17/123 expected failures
 - /home/vagrant/pypy/bin/python -u test__backdoor.py
 - /home/vagrant/pypy/bin/python -u test__issue6.py
 - /home/vagrant/pypy/bin/python -u -m monkey_test --Event test_threading.py
 - /home/vagrant/pypy/bin/python -u test__threading_vs_settrace.py
 - /home/vagrant/pypy/bin/python -u -m monkey_test test_ssl.py
 - /home/vagrant/pypy/bin/python -u test__refcount.py
 - /home/vagrant/pypy/bin/python -u test__pywsgi.py
 - /home/vagrant/pypy/bin/python -u -m monkey_test --Event test_subprocess.py
 - /home/vagrant/pypy/bin/python -u -m monkey_test --Event test_socket.py
 - /home/vagrant/pypy/bin/python -u test__core_stat.py
 - /home/vagrant/pypy/bin/python -u -m monkey_test test_threading.py
 - /home/vagrant/pypy/bin/python -u test__server.py
 - /home/vagrant/pypy/bin/python -u test_threading_2.py
 - /home/vagrant/pypy/bin/python -u -m monkey_test test_socket.py
 - /home/vagrant/pypy/bin/python -u test__ares_host_result.py
 - /home/vagrant/pypy/bin/python -u -m monkey_test test_subprocess.py
 - /home/vagrant/pypy/bin/python -u -m monkey_test --Event test_ssl.py

2/123 unexpected failures
 - /home/vagrant/pypy/bin/python -u -m monkey_test --Event test_httpservers.py
 - /home/vagrant/pypy/bin/python -u -m monkey_test test_httpservers.py
```

And OSX:

```
2/123 unexpected passes
 - /Users/jay/other-projects/gevent/env/bin/python -u test__real_greenlet.py
 - /Users/jay/other-projects/gevent/env/bin/python -u test__exc_info.py

21/123 tests failed in 03:26

17/123 expected failures
 - /Users/jay/other-projects/gevent/env/bin/python -u test__threading_vs_settrace.py
 - /Users/jay/other-projects/gevent/env/bin/python -u -m monkey_test --Event test_ssl.py
 - /Users/jay/other-projects/gevent/env/bin/python -u -m monkey_test test_subprocess.py
 - /Users/jay/other-projects/gevent/env/bin/python -u test__core_stat.py
 - /Users/jay/other-projects/gevent/env/bin/python -u test__backdoor.py
 - /Users/jay/other-projects/gevent/env/bin/python -u test__issue6.py
 - /Users/jay/other-projects/gevent/env/bin/python -u test__pywsgi.py
 - /Users/jay/other-projects/gevent/env/bin/python -u -m monkey_test test_ssl.py
 - /Users/jay/other-projects/gevent/env/bin/python -u test_threading_2.py
 - /Users/jay/other-projects/gevent/env/bin/python -u -m monkey_test --Event test_socket.py
 - /Users/jay/other-projects/gevent/env/bin/python -u test__ares_host_result.py
 - /Users/jay/other-projects/gevent/env/bin/python -u -m monkey_test test_threading.py
 - /Users/jay/other-projects/gevent/env/bin/python -u test__refcount.py
 - /Users/jay/other-projects/gevent/env/bin/python -u -m monkey_test --Event test_threading.py
 - /Users/jay/other-projects/gevent/env/bin/python -u test__server.py
 - /Users/jay/other-projects/gevent/env/bin/python -u -m monkey_test --Event test_subprocess.py
 - /Users/jay/other-projects/gevent/env/bin/python -u -m monkey_test test_socket.py

4/123 unexpected failures
 - /Users/jay/other-projects/gevent/env/bin/python -u -m monkey_test --Event test_urllib2.py
 - /Users/jay/other-projects/gevent/env/bin/python -u -m monkey_test --Event test_httpservers.py
 - /Users/jay/other-projects/gevent/env/bin/python -u -m monkey_test test_httpservers.py
 - /Users/jay/other-projects/gevent/env/bin/python -u test__socket_dns6.py
```