Fix AttributeError in add_new_check.py

[j-carl]

Summary

Using add_new_check.py with Python 3.8.2 to create a new check results
in a AttributeError and an empty file <module>/<Module>TidyModule.cpp.

Python 3.x doesn't support next() anymore and should be replace with
__next__().

$ ./add_new_check.py readability test-flag
Updating ./readability/CMakeLists.txt...
Creating ./readability/TestFlagCheck.h...
Creating ./readability/TestFlagCheck.cpp...
Updating ./readability/ReadabilityTidyModule.cpp...
Traceback (most recent call last):

File "./add_new_check.py", line 471, in <module>
  main()
File "./add_new_check.py", line 461, in main
  adapt_module(module_path, module, check_name, check_name_camel)
File "./add_new_check.py", line 175, in adapt_module
  line = lines.next()

AttributeError: 'list_iterator' object has no attribute 'next'

clang-tools-extra/clang-tidy/add_new_check.py

with open(filename, 'w') as f:
     check_added = False
     check_fq_name = module + '-' + check_name
     check_decl = ('    CheckFactories.registerCheck<' + check_name_camel +
                   '>(\n        "' + check_fq_name + '");\n')
 
     lines = iter(lines)
     try:
       while True:
-        line = lines.next()
+        line = lines.__next__()
         if not header_added:
           match = re.search('#include "(.*)"', line)
           if match:
             header_found = True
             if match.group(1) > check_name_camel:
               header_added = True
               f.write('#include "' + check_name_camel + '.h"\n')
           elif header_found:
             header_added = True
             f.write('#include "' + check_name_camel + '.h"\n')
 
         if not check_added:
           if line.strip() == '}':
             check_added = True
             f.write(check_decl)
           else:
             match = re.search('registerCheck<(.*)> *\( *(?:"([^"]*)")?', line)
             prev_line = None
             if match:
               current_check_name = match.group(2)
               if current_check_name is None:
                 # If we didn't find the check name on this line, look on the
                 # next one.
                 prev_line = line
-                line = lines.next()
+                line = lines.__next__()
                 match = re.search(' *"([^"]*)"', line)
                 if match:
                   current_check_name = match.group(1)
               if current_check_name > check_fq_name:
                 check_added = True
                 f.write(check_decl)
               if prev_line:
                 f.write(prev_line)