Fix tests on windows - shebang don't work so great there!

test/13_subdir_package/bin/before_build.py

@@ -0,0 +1 @@
+print('before_build.py executed!')

test/13_subdir_package/bin/before_build.sh

@@ -1,3 +0,0 @@
-#!/bin/bash
-
-echo 'before_build.sh executed.'

test/13_subdir_package/cibuildwheel_test.py

@@ -8,7 +8,7 @@ def test():
     package_dir = os.path.join(project_dir, 'src', 'spam')
     # build the wheels
     actual_wheels = utils.cibuildwheel_run(project_dir, package_dir=package_dir, add_env={
-        'CIBW_BEFORE_BUILD': '{project}/bin/before_build.sh',
+        'CIBW_BEFORE_BUILD': 'python {project}/bin/before_build.py',
         'CIBW_TEST_COMMAND': 'python {package}/test/run_tests.py',
         # this shouldn't depend on the version of python, so build only
         # CPython 3.6

test/13_subdir_package/src/spam/test/run_tests.py

@@ -1 +1 @@
-print('run_tests.py called!')
+print('run_tests.py executed!')