Auto merge of #6996 - frewsxcv:prevent-tidy-crash, r=jdm

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.

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