Protection against ReDoS

[stsewd]

The regex module is compatible with the re module (VERSION0 flag).
It is also faster.

>>> import re
>>> import regex
>>> import timeit
>>> pattert = "(a+)+b"
>>> input = "a" * 25
>>> timeit.timeit(lambda: re.search(pattern, input), number=10)
32.332445038000515
>>> timeit.timeit(lambda: regex.search(pattern, input, flags=regex.VERSION0), number=10)
0.003861578001306043
>>> input = "a" * 10000
>>> regex.search(pattern, input, flags=regex.VERSION0, timeout=5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/stsewd/.pyenv/versions/readthedocs.org/lib/python3.6/site-packages/regex/regex.py", line 266, in search
    concurrent, partial, timeout)
TimeoutError: regex timed out

I put the timeout to 15, maybe we can drop it to 5?

[humitos]

This PR is related to #5996.

[humitos]

We decided to ship with regex (#4641 (comment)) so we should merge this PR before that PR gets merged, or merge this PR into the other first.

[humitos]

I put the timeout to 15, maybe we can drop it to 5?

Even less, should be better. Parsing a regex shouldn't take more than 1s.

[stsewd]

Ok, I've decreased the timeout to 1 second. Another alternative is to use a finite state machine type of regex, but I wasn't able to find one lib for python...