bpo-37253: Add _PyCompilerFlags_INIT macro (GH-14018)

Add a new _PyCompilerFlags_INIT macro to initialize PyCompilerFlags
variables, rather than initializing cf_flags and cf_feature_version
explicitly in each variable.
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index 784c15b..8f3ee19 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -91,7 +91,7 @@
 {
     PyObject *filename, *v;
     int ret, err;
-    PyCompilerFlags local_flags;
+    PyCompilerFlags local_flags = _PyCompilerFlags_INIT;
     int nomem_count = 0;
 #ifdef Py_REF_DEBUG
     int show_ref_count = _PyInterpreterState_Get()->config.show_ref_count;
@@ -105,8 +105,6 @@
 
     if (flags == NULL) {
         flags = &local_flags;
-        local_flags.cf_flags = 0;
-        local_flags.cf_feature_version = PY_MINOR_VERSION;
     }
     v = _PySys_GetObjectId(&PyId_ps1);
     if (v == NULL) {
@@ -1283,10 +1281,7 @@
 struct symtable *
 Py_SymtableStringObject(const char *str, PyObject *filename, int start)
 {
-    PyCompilerFlags flags;
-
-    flags.cf_flags = 0;
-    flags.cf_feature_version = PY_MINOR_VERSION;
+    PyCompilerFlags flags = _PyCompilerFlags_INIT;
     return _Py_SymtableStringObjectFlags(str, filename, start, &flags);
 }
 
@@ -1331,7 +1326,7 @@
                              PyCompilerFlags *flags, PyArena *arena)
 {
     mod_ty mod;
-    PyCompilerFlags localflags;
+    PyCompilerFlags localflags = _PyCompilerFlags_INIT;
     perrdetail err;
     int iflags = PARSER_FLAGS(flags);
     if (flags && flags->cf_feature_version < 7)
@@ -1341,8 +1336,6 @@
                                          &_PyParser_Grammar, start, &err,
                                          &iflags);
     if (flags == NULL) {
-        localflags.cf_flags = 0;
-        localflags.cf_feature_version = PY_MINOR_VERSION;
         flags = &localflags;
     }
     if (n) {
@@ -1379,7 +1372,7 @@
                            PyArena *arena)
 {
     mod_ty mod;
-    PyCompilerFlags localflags;
+    PyCompilerFlags localflags = _PyCompilerFlags_INIT;
     perrdetail err;
     int iflags = PARSER_FLAGS(flags);
 
@@ -1387,8 +1380,6 @@
                                        &_PyParser_Grammar,
                                        start, ps1, ps2, &err, &iflags);
     if (flags == NULL) {
-        localflags.cf_flags = 0;
-        localflags.cf_feature_version = PY_MINOR_VERSION;
         flags = &localflags;
     }
     if (n) {