static allocation for PyModuleDef, to portably avoid leak check errors.

This PR solves the same issue as pybind#2019 (rolled back), but in a way that is
certain to be portable and will work for any leak checker.

The Python 3 documentation suggests `static` allocation for `PyModuleDef`:

  * https://docs.python.org/3/c-api/module.html#initializing-c-modules

  * The module definition struct, which holds all information needed to
    create a module object. There is usually only one statically initialized
    variable of this type for each module.

This PR changes the `PYBIND11_MODULE` macro accordingly: `static PyModuleDef mdef;`

The `pybind11::module::module` code is slightly refactored, with the idea
to make the future removal of Python 2 support straightforward.

Author: rwgk

include/pybind11/detail/common.h

@@ -307,13 +307,21 @@ extern "C" {
             });
         }
 \endrst */
+#if PY_MAJOR_VERSION >= 3
+#define PYBIND11_MODULE_PY2_VS_PY3_SPECIFIC(name)                              \
+        static PyModuleDef mdef;                                               \
+        auto m = pybind11::module(PYBIND11_TOSTRING(name), nullptr, &mdef);
+#else
+#define PYBIND11_MODULE_PY2_VS_PY3_SPECIFIC(name)                              \
+        auto m = pybind11::module(PYBIND11_TOSTRING(name));
+#endif
 #define PYBIND11_MODULE(name, variable)                                        \
     PYBIND11_MAYBE_UNUSED                                                      \
     static void PYBIND11_CONCAT(pybind11_init_, name)(pybind11::module &);     \
     PYBIND11_PLUGIN_IMPL(name) {                                               \
         PYBIND11_CHECK_PYTHON_VERSION                                          \
         PYBIND11_ENSURE_INTERNALS_READY                                        \
-        auto m = pybind11::module(PYBIND11_TOSTRING(name));                    \
+        PYBIND11_MODULE_PY2_VS_PY3_SPECIFIC(name)                              \
         try {                                                                  \
             PYBIND11_CONCAT(pybind11_init_, name)(m);                          \
             return m.ptr();                                                    \

include/pybind11/pybind11.h

@@ -857,18 +857,18 @@ class module : public object {
     PYBIND11_OBJECT_DEFAULT(module, object, PyModule_Check)
 
     /// Create a new top-level Python module with the given name and docstring
-    explicit module(const char *name, const char *doc = nullptr) {
-        if (!options::show_user_defined_docstrings()) doc = nullptr;
-#if PY_MAJOR_VERSION >= 3
-        auto *def = new PyModuleDef();
+    explicit module(const char *name, const char *doc = nullptr, PyModuleDef *def = nullptr) {
+        if (!def) def = new PyModuleDef();
         std::memset(def, 0, sizeof(PyModuleDef));
         def->m_name = name;
-        def->m_doc = doc;
+        def->m_doc = options::show_user_defined_docstrings() ? doc : nullptr;
         def->m_size = -1;
         Py_INCREF(def);
         m_ptr = PyModule_Create(def);
 #else
-        m_ptr = Py_InitModule3(name, nullptr, doc);
+    explicit module(const char *name, const char *doc = nullptr) {
+        m_ptr = Py_InitModule3(
+            name, nullptr, options::show_user_defined_docstrings() ? doc : nullptr);
 #endif
         if (m_ptr == nullptr)
             pybind11_fail("Internal error in module::module()");