[Pygments] Complete stubs for Formatter (#15608)

stubs/Pygments/@tests/stubtest_allowlist.txt

@@ -14,6 +14,10 @@ pygments.lexer.Lexer.name
 pygments.lexer.Lexer.url
 pygments.lexer.Lexer.version_added
 
+# Class attributes that are set to None in the base class, but are
+# always overridden with a non-None value in subclasses.
+pygments.formatter.Formatter.name
+
 # Individual lexers and styles submodules are not stubbed at this time.
 pygments\.lexers\.(?!get_|find_|load_|guess_).*
 pygments\.styles\.(?!get_).*