Prevent flake8 from crashing tidy

If someone were to write:

    def hello() :
        print "hello world"

flake8 would warn:

    stdin:54:45: E203 whitespace before ':'

Normally there are only three colons in a flake8 error message, but this
one has four, which causes issue with this line:

    _, line_num, _, message = error.split(":")

...causing this error:

    ValueError: too many values to unpack

This commit updates the `str.split` call to utilize the `maxsplit`
parameter to prevent this error from occurring.

python/tidy.py

@@ -153,7 +153,7 @@ def check_flake8(file_name, contents):
     with stdout_redirect(output):
         check_code(contents, ignore=ignore)
     for error in output.getvalue().splitlines():
-        _, line_num, _, message = error.split(":")
+        _, line_num, _, message = error.split(":", 3)
         yield line_num, message.strip()