Automatically add stubs for "cython.cimports.*" in the test runner to make...

Automatically add stubs for "cython.cimports.*" in the test runner to make them importable in Python (although not necessarily runnable).

docs/examples/tutorial/pure/py_cimport.py

+
+from cython.cimports.libc import math
+
+def use_libc_math():
+    return math.ceil(5.5)

docs/src/tutorial/pure.rst

@@ -218,11 +218,7 @@ libraries become available to Python code.  It only means that you can
 tell Cython what cimports you want to use, without requiring special
 syntax.  Running such code in plain Python will fail.
 
-::
-
-    from cython.cimports.libc import math
-
-    print(math.ceil(5.5))
+.. literalinclude:: ../../examples/tutorial/pure/py_cimport.py
 
 Since such code must necessarily refer to the non-existing
 ``cython.cimports`` 'package', the plain cimport form

runtests.py

@@ -980,6 +980,15 @@ class CythonCompileTestCase(unittest.TestCase):
         if self.workdir not in sys.path:
             sys.path.insert(0, self.workdir)
 
+        if self.add_cython_import:
+            with open(self.module_path, 'rb') as f:
+                source = f.read()
+                if b'cython.cimports.' in source:
+                    from Cython.Shadow import CythonCImports
+                    for name in set(re.findall(br"(cython\.cimports(?:\.\w+)+)", source)):
+                        name = name.decode()
+                        sys.modules[name] = CythonCImports(name)
+
     def tearDown(self):
         from Cython.Compiler import Options
         for name, value in self._saved_options:
@@ -995,6 +1004,13 @@ class CythonCompileTestCase(unittest.TestCase):
             del sys.modules[self.module]
         except KeyError:
             pass
+
+        # remove any stubs of cimported modules in pure Python mode
+        if self.add_cython_import:
+            for name in list(sys.modules):
+                if name.startswith('cython.cimports.'):
+                    del sys.modules[name]
+
         cleanup = self.cleanup_failures or self.success
         cleanup_c_files = WITH_CYTHON and self.cleanup_workdir and cleanup
         cleanup_lib_files = self.cleanup_sharedlibs and cleanup