Have mkstringprep.py download the RFC if it's missing

Author: encukou

Tools/unicode/mkstringprep.py

@@ -1,7 +1,11 @@
 import re
+import os
 import unicodedata as unicodedata_current
 from unicodedata import ucd_3_2_0 as unicodedata_320
 
+FILENAME = "Tools/unicode/data/rfc3454.txt"
+URL = "https://www.rfc-editor.org/rfc/rfc3454.txt"
+
 def gen_category(cats):
     for i in range(0, 0x110000):
         if unicodedata_320.category(chr(i)) in cats:
@@ -48,8 +52,16 @@ def compact_set(l):
 
 ############## Read the tables in the RFC #######################
 
-with open("rfc3454.txt") as f:
-    data = f.readlines()
+try:
+    data_file = open(FILENAME, encoding='utf-8')
+except FileNotFoundError:
+    import urllib.request
+    os.makedirs(os.path.dirname(FILENAME), exist_ok=True)
+    urllib.request.urlretrieve(URL, filename=FILENAME)
+    data_file = open(FILENAME, encoding='utf-8')
+
+with data_file:
+    data = data_file.readlines()
 
 tables = []
 curname = None