bpo-45434: Convert Py_GETENV() macro to a function (GH-28912)

Avoid calling directly getenv() in the header file.

Include/cpython/pydebug.h

@@ -29,7 +29,7 @@ PyAPI_DATA(int) Py_LegacyWindowsStdioFlag;
 /* this is a wrapper around getenv() that pays attention to
    Py_IgnoreEnvironmentFlag.  It should be used for getting variables like
    PYTHONPATH and PYTHONHOME from the environment */
-#define Py_GETENV(s) (Py_IgnoreEnvironmentFlag ? NULL : getenv(s))
+PyAPI_DATA(char*) Py_GETENV(const char *name);
 
 #ifdef __cplusplus
 }

Python/initconfig.c

@@ -249,6 +249,14 @@ fail:
 #undef SET_ITEM_STR
 }
 
+char*
+Py_GETENV(const char *name)
+{
+    if (Py_IgnoreEnvironmentFlag) {
+        return NULL;
+    }
+    return getenv(name);
+}
 
 /* --- PyStatus ----------------------------------------------- */