Fix single quote escaping for python code generator (and its test) (#21021)

Hard to spot, but in Java the strings "'" and "\'" are identical, so
the implementation and its test were broken identically.

Author: curdbecker

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractPythonCodegen.java

@@ -214,7 +214,7 @@ public String toDefaultValue(Schema p) {
                 String defaultValue = String.valueOf(p.getDefault());
                 if (defaultValue != null) {
                     defaultValue = defaultValue.replace("\\", "\\\\")
-                            .replace("'", "\'");
+                            .replace("'", "\\'");
                     if (Pattern.compile("\r\n|\r|\n").matcher(defaultValue).find()) {
                         return "'''" + defaultValue + "'''";
                     } else {

modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonClientCodegenTest.java

@@ -139,7 +139,7 @@ public void testSingleQuotes() {
         StringSchema schema = new StringSchema();
         schema.setDefault("Text containing 'single' quote");
         String defaultValue = codegen.toDefaultValue(schema);
-        Assert.assertEquals("'Text containing \'single\' quote'", defaultValue);
+        Assert.assertEquals("'Text containing \\'single\\' quote'", defaultValue);
     }
 
     @Test(description = "test backslash default")