Switch from using pep8 to pycodestyle

[martinholmer]

This pull request follows the suggestion printed by pep8 version 1.7.1 and switches from using pep8 to pycodestyle. Here is the warning printed by version 1.7.1 of pep8:

tax-calculator$ pep8 taxcalc
/Users/mrh/anaconda2/lib/python2.7/site-packages/pep8.py:2124: UserWarning: 

pep8 has been renamed to pycodestyle (GitHub issue #466)
Use of the pep8 tool will be removed in a future release.
Please install and use `pycodestyle` instead.

$ pip install pycodestyle
$ pycodestyle ...

tax-calculator$ pep8 --version
/Users/mrh/anaconda2/lib/python2.7/site-packages/pep8.py:2124: UserWarning: 

pep8 has been renamed to pycodestyle (GitHub issue #466)
Use of the pep8 tool will be removed in a future release.
Please install and use `pycodestyle` instead.

$ pip install pycodestyle
$ pycodestyle ...

1.7.1

This pull request continues to use pep8 on GitHub because a needed package --- pytest-codestyle --- is not available as a conda package.

[codecov-io]

Codecov Report

Merging #1989 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1989   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          38      38           
  Lines        3756    3756           
======================================
  Hits         3756    3756

Impacted Files	Coverage Δ
taxcalc/parameters.py	100% <ø> (ø)	⬆️
taxcalc/growdiff.py	100% <ø> (ø)	⬆️
taxcalc/simpletaxio.py	100% <ø> (ø)	⬆️
taxcalc/records.py	100% <ø> (ø)	⬆️
taxcalc/policy.py	100% <ø> (ø)	⬆️
taxcalc/decorators.py	100% <ø> (ø)	⬆️
taxcalc/utilsprvt.py	100% <ø> (ø)	⬆️
taxcalc/taxcalcio.py	100% <ø> (ø)	⬆️
taxcalc/utils.py	100% <ø> (ø)	⬆️
taxcalc/growfactors.py	100% <ø> (ø)	⬆️
... and 4 more

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