Issue 16152: Trailing whitespace makes tokenize.generate_tokens pathological

When tokenizing with tokenize.generate_tokens, if the code ends with whitespace (no newline), the tokenizer produces an ERRORTOKEN for each space.  Additionally, the regex that fails to find tokens in those spaces is linear in the number of spaces, so the overall performance is O(n**2).

I found this while tokenizing code samples uploaded to a public web site.  One sample for some reason ended with 40,000 spaces, which was taking two hours to tokenize.

Demonstration:

{{{
import token
import tokenize

try:
    from cStringIO import StringIO
except:
    from io import StringIO

code = "@"+" "*10000
code_reader = StringIO(code).readline

for num, (ttyp, ttok, _, _, _) in enumerate(tokenize.generate_tokens(code_reader)):
    print("%5d %15s %r" % (num, token.tok_name[ttyp], ttok))
}}}