use tokenize.open to avoid incorrect line endings at beginning of file

[asottile]

Relates to #974

[asottile]

note that the testsuite failed before this with:

$ tox -e py37
GLOB sdist-make: /tmp/z/pycodestyle/setup.py
py37 create: /tmp/z/pycodestyle/.tox/py37
py37 installdeps: coverage
py37 inst: /tmp/z/pycodestyle/.tox/.tmp/package/1/pycodestyle-2.6.0.zip
py37 installed: coverage==5.3.1,pycodestyle @ file:///tmp/z/pycodestyle/.tox/.tmp/package/1/pycodestyle-2.6.0.zip
py37 run-test-pre: PYTHONHASHSEED='3100982836'
py37 run-test: commands[0] | python -m pycodestyle --statistics pycodestyle.py
py37 run-test: commands[1] | coverage run -m pycodestyle --max-doc-length=72 --testsuite testsuite
'estsuite/crlf.py:1:4: W605:1:4 invalid escape sequence '\
3864 lines tested: 40 files, 442 test cases, 1 failures.
Test failed.
ERROR: InvocationError for command /tmp/z/pycodestyle/.tox/py37/bin/coverage run -m pycodestyle --max-doc-length=72 --testsuite testsuite (exited with code 1)
________________________________________ summary ________________________________________
ERROR:   py37: commands failed