Allow Unicode in Python identifiers (rouge-ruby#1510)

Python supports the use of Unicode in identifiers. This commit uses
POSIX bracket expressions that match against Unicode characters rather
than the more common character classes that only match ASCII characters.

Co-authored-by: Michael Camilleri <mike@inqk.net>

lib/rouge/lexers/python.rb

@@ -67,8 +67,8 @@ def self.exceptions
         )
       end
 
-      identifier =        /[a-z_][a-z0-9_]*/i
-      dotted_identifier = /[a-z_.][a-z0-9_.]*/i
+      identifier =        /[[:alpha:]_][[:alnum:]_]*/
+      dotted_identifier = /[[:alpha:]_.][[:alnum:]_.]*/
 
       def current_string
         @string_register ||= StringRegister.new

spec/visual/samples/python

@@ -150,3 +150,8 @@ x @= y
 f'{hello} world {int(x) + 1}'
 f'{{ {4*10} }}'
 f'result: {value:{width}.{precision}}'
+
+# Unicode identifiers
+α = 10
+def coöperative(б):
+    return f"{б} is Russian"