pyx.build: Fix support for develop install `pip install -e` (v↑ setuptools_dso (1.6 -> 1.7))

Starting from setuptools-dso 1.7 pyx.build no longer misses to
(re)compile dsos when `build_ext -i` is run:

https://github.com/mdavidsaver/setuptools_dso/pull/10
https://github.com/mdavidsaver/setuptools_dso/commit/84e06348

Previously `pip install -e` would just fail when run afresh because DSOs
were not built, and so link ext->dso was failing.

pyproject.toml

 [build-system]
-requires = ["setuptools", "wheel", "setuptools_dso >= 1.6", "cython", "gevent"]
+requires = ["setuptools", "wheel", "setuptools_dso >= 1.7", "cython", "gevent"]

setup.py

@@ -156,7 +156,7 @@ class develop(XInstallGPython, _develop):
 # requirements of packages under "golang." namespace
 R = {
     'cmd.pybench':      {'pytest'},
-    'pyx.build':        {'setuptools', 'wheel', 'cython', 'setuptools_dso >= 1.6'},
+    'pyx.build':        {'setuptools', 'wheel', 'cython', 'setuptools_dso >= 1.7'},
     'x.perf.benchlib':  {'numpy'},
 }
 # TODO generate `a.b -> a`, e.g. x.perf = join(x.perf.*); x = join(x.*)