Fix version detection and compile/runtime Python version warning with...

Fix version detection and compile/runtime Python version warning with double-digit versions such as Py3.10 (GH-4777)

See https://github.com/cython/cython/issues/2542#issuecomment-1121484721

Cython/Utility/ModuleSetupCode.c

@@ -1223,11 +1223,37 @@ static int __Pyx_check_binary_version(void);
 /////////////// CheckBinaryVersion ///////////////
 
 static int __Pyx_check_binary_version(void) {
-    char ctversion[4], rtversion[4];
-    PyOS_snprintf(ctversion, 4, "%d.%d", PY_MAJOR_VERSION, PY_MINOR_VERSION);
-    PyOS_snprintf(rtversion, 4, "%s", Py_GetVersion());
-    if (ctversion[0] != rtversion[0] || ctversion[2] != rtversion[2]) {
+    char ctversion[5];
+    int same=1, i, found_dot;
+    const char* rt_from_call = Py_GetVersion();
+    PyOS_snprintf(ctversion, 5, "%d.%d", PY_MAJOR_VERSION, PY_MINOR_VERSION);
+    // slightly convoluted, but now that we're into double digit version numbers we can no longer just rely on the length.
+    found_dot = 0;
+    for (i = 0; i < 4; i++) {
+        if (!ctversion[i]) {
+            // if they are the same, just check that the runtime version doesn't continue with further numbers
+            same = (rt_from_call[i] < '0' || rt_from_call[i] > '9');
+            break;
+        }
+        if (rt_from_call[i] != ctversion[i]) {
+            same = 0;
+            break;
+        }
+    }
+
+    if (!same) {
+        char rtversion[5] = {'\0'};
+        // copy the runtime-version for the error message
         char message[200];
+        for (i=0; i<4; ++i) {
+            if (rt_from_call[i] == '.') {
+                if (found_dot) break;
+                found_dot = 1;
+            } else if (rt_from_call[i] < '0' || rt_from_call[i] > '9') {
+                break;
+            }
+            rtversion[i] = rt_from_call[i];
+        }
         PyOS_snprintf(message, sizeof(message),
                       "compiletime version %s of module '%.100s' "
                       "does not match runtime version %s",