setup: py.bench needs py on py3

Because recent pytest stoppped to depend on py and py.bench fails without py installed:

    (py3.venv) kirr@deca:~/src/tools/go/pygolang-master$ py.bench golang/strconv_test.py
    Traceback (most recent call last):
      File "/home/kirr/src/tools/go/py3.venv/bin/py.bench", line 5, in <module>
        from golang.cmd.pybench import main
      File "/home/kirr/src/tools/go/pygolang-master/golang/cmd/pybench.py", line 57, in <module>
        from py.process import ForkedFunc
    ModuleNotFoundError: No module named 'py.process'; 'py' is not a package

/proposed-for-review-on nexedi/pygolang!25

setup.py

@@ -154,7 +154,7 @@ class develop(XInstallGPython, _develop):
 
 # requirements of packages under "golang." namespace
 R = {
-    'cmd.pybench':      {'pytest'},
+    'cmd.pybench':      {'pytest', 'py ; python_version >= "3"'},
     'pyx.build':        {'setuptools', 'wheel', 'cython < 3', 'setuptools_dso >= 2.8'},
     'x.perf.benchlib':  {'numpy'},
 }