Starting from gevent >= 1.5 '*.pxd' files for gevent API are no longer
provided, at least in released gevent wheels. This broke pygolang:

    Error compiling Cython file:
    ------------------------------------------------------------
    ...

    # Gevent runtime uses gevent's greenlets and semaphores.
    # When sema.acquire() blocks, gevent switches us from current to another greenlet.

    IF not PYPY:
        from gevent._greenlet cimport Greenlet
       ^
    ------------------------------------------------------------

    golang/runtime/_runtime_gevent.pyx:28:4: 'gevent/_greenlet.pxd' not found

Since gevent upstream refuses to restore Cython level access[1], let's fix the
build by using gevent bits via Python-level.

Even when used via py import gevent-1.5 brings speed improvement compared to
gevent-1.4 (used via cimport):

	(on i7@2.6GHz, gevent runtime)

                      gevent-1.4   gevent-1.5
                      (cimport)    (py import)

    name              old time/op  new time/op  delta
    pyx_select_nogil  9.47µs ± 0%  8.74µs ± 0%   -7.70%  (p=0.000 n=10+9)
    pyx_go_nogil      14.3µs ± 1%  12.0µs ± 1%  -16.52%  (p=0.000 n=10+10)
    pyx_chan_nogil    7.10µs ± 1%  6.32µs ± 1%  -10.89%  (p=0.000 n=10+10)
    go                16.0µs ± 2%  13.4µs ± 1%  -16.37%  (p=0.000 n=10+10)
    chan              7.50µs ± 0%  6.79µs ± 0%   -9.53%  (p=0.000 n=10+10)
    select            10.8µs ± 1%  10.0µs ± 1%   -6.78%  (p=0.000 n=10+10)

Using gevent-1.5 could have been even faster via cimport (it is still
possible to compile and test against gevent installed in development
mode via `pip install -e` because pxd files are there in gevent worktree
and tarball):

                      gevent-1.5   gevent-1.5
                      (py import)  (cimport)

    name              old time/op  new time/op  delta
    pyx_select_nogil  8.74µs ± 0%  7.90µs ± 1%  -9.60%  (p=0.000 n=9+10)
    pyx_go_nogil      12.0µs ± 1%  11.2µs ± 2%  -6.35%  (p=0.000 n=10+10)
    pyx_chan_nogil    6.32µs ± 1%  5.89µs ± 0%  -6.80%  (p=0.000 n=10+9)
    go                13.4µs ± 1%  12.4µs ± 1%  -7.54%  (p=0.000 n=10+9)
    chan              6.79µs ± 0%  6.42µs ± 0%  -5.47%  (p=0.000 n=10+10)
    select            10.0µs ± 1%   9.4µs ± 1%  -6.39%  (p=0.000 n=10+10)

but we cannot use cimport to access gevent-1.5 universally, since pxd are not
shipped in gevent wheel releases.

In the future we might want to change plain version check into compile time
check whether gevent/_greenlet.pxd is actually present or not and use faster
access if yes. Requesting gevent to be installed in non-binary form
might be also an option worth trying.

However plain version check should be ok for now.

[1] https://github.com/gevent/gevent/issues/1568