Add before_build option, that runs a shell command before 'pip wheel'

test/03_before_build/setup.py

@@ -0,0 +1,17 @@
+from setuptools import setup, Extension
+import sys
+
+# here we assert that the Python version as written to version.txt in the CIBW_BEFORE_BUILD step
+# is the same one as is currently running.
+sys.stderr.write('sys.argv \n' + str(sys.argv))
+if sys.argv[-1] != '--name':
+    with open('version.txt') as f:
+        stored_version = f.read()
+    print('version is', stored_version)
+    assert stored_version == sys.version
+
+setup(
+    name="spam",
+    ext_modules=[Extension('spam', sources=['spam.c'])],
+    version="0.1.0",
+)