fix(python) identifiers starting with underscore not highlighted

Since hljs version 11, python identifiers starting with an underscore
character were no longer highlighted (regression introduced in 952fa0a).

Add new python markup test related to identifiers highlighting.

src/languages/python.js

@@ -5,7 +5,7 @@ Website: https://www.python.org
 Category: common
 */
 
-import { IDENT_RE } from '../lib/modes.js';
+import { UNDERSCORE_IDENT_RE } from '../lib/modes.js';
 import * as regex from '../lib/regex.js';
 
 export default function(hljs) {
@@ -379,7 +379,7 @@ export default function(hljs) {
       {
         match: [
           /def/, /\s+/,
-          IDENT_RE
+          UNDERSCORE_IDENT_RE
         ],
         scope: {
           1: "keyword",
@@ -392,14 +392,14 @@ export default function(hljs) {
           {
             match: [
               /class/, /\s+/,
-              IDENT_RE, /\s*/,
-              /\(\s*/, IDENT_RE,/\s*\)/
+              UNDERSCORE_IDENT_RE, /\s*/,
+              /\(\s*/, UNDERSCORE_IDENT_RE,/\s*\)/
             ],
           },
           {
             match: [
               /class/, /\s+/,
-              IDENT_RE
+              UNDERSCORE_IDENT_RE
             ],
           }
         ],

test/markup/python/identifiers.expect.txt

@@ -0,0 +1,12 @@
+<span class="hljs-keyword">def</span> <span class="hljs-title function_">func</span>():
+  <span class="hljs-keyword">pass</span>
+
+<span class="hljs-keyword">def</span> <span class="hljs-title function_">_private_func</span>():
+  <span class="hljs-keyword">pass</span>
+
+<span class="hljs-keyword">class</span> <span class="hljs-title class_">IdentifiersTest</span>:
+  <span class="hljs-keyword">def</span> <span class="hljs-title function_">__init__</span>(<span class="hljs-params">self</span>):
+    <span class="hljs-keyword">pass</span>
+
+  <span class="hljs-keyword">def</span> <span class="hljs-title function_">method</span>(<span class="hljs-params">self</span>):
+    <span class="hljs-keyword">pass</span>

test/markup/python/identifiers.txt

@@ -0,0 +1,13 @@
+def func():
+  pass
+
+def _private_func():
+  pass
+
+class IdentifiersTest:
+  def __init__(self):
+    pass
+
+  def method(self):
+    pass
+