fix import statement moved above the module doc by E402

[Hanaasagi]

This pull request resolves that import statement moved above the module document when fixing E402. I don't want to use the pycodestyle.DOCSTRING_REGEX to check whether this line is a docstring. Because pycodestyle implement it as:

DOCSTRING_REGEX = re.compile(r'u?r?["\']')

In PEP 257 -- Docstring Conventions

For consistency, always use """triple double quotes""" around docstrings. Use r"""raw triple double quotes""" if you use any backslashes in your docstrings. For Unicode docstrings, use u"""Unicode triple-quoted strings""".

So, I add a regex DOCSTRING_START_REGEX to match the module doc prefix.

resolves #459

[coveralls]

Coverage decreased (-0.01%) to 98.67% when pulling 59318cb on Hanaasagi:fix/import-statement-moved-above-docstring-by-E402 into 89886b4 on hhatto:master.

[coveralls]

Coverage decreased (-0.01%) to 98.67% when pulling 59318cb on Hanaasagi:fix/import-statement-moved-above-docstring-by-E402 into 89886b4 on hhatto:master.

[coveralls]

Coverage decreased (-0.01%) to 98.67% when pulling 59318cb on Hanaasagi:fix/import-statement-moved-above-docstring-by-E402 into 89886b4 on hhatto:master.

[hhatto]

Cool!!

LGTM 👍