Add PEP8_FLAGS variable to the test suite Python makefile.

Put pep8 options into this variable to avoid repeating them twice.

No real changes.
Ignore E402 (import not on top of file) PEP8 error.

Travis uses the latest pep8 sources from Git and since

https://github.com/jcrocholl/pep8/commit/f3a12babd4278f8f7a529a9d1d63d56faf071cf8

this error is given for all Python files generated by SWIG with -builtin
option.
Merge pull request #276 from vadz/pep8

Ignore another pep8 error.