Use string instead of writing alphabets manually (#3)

* add

* Update paths.py

Author: baseplate-admin

django_ltree/paths.py

@@ -1,17 +1,18 @@
+import string
 from itertools import product
 
 from django_ltree.fields import PathValue
 
 
 class PathGenerator(object):
-    _alphabet = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
     _default_label_size = 6  # Postgres limits this to 256
 
     def __init__(self, prefix=None, skip=None, label_size=None):
         self.skip_paths = [] if skip is None else skip[:]
         self.path_prefix = prefix if prefix else []
         self.product_iterator = product(
-            self._alphabet, repeat=label_size or self._default_label_size
+            string.digits + string.ascii_letters,
+            repeat=label_size or self._default_label_size,
         )
 
     def __iter__(self):