generateetags.py: use raw string to avoid warning on macOS
$ make alletags
/Users/jroman/soft/petsc/lib/petsc/bin/maint/generateetags.py:176: SyntaxWarning: invalid escape sequence '\('
output = check_output('git ls-files | grep -E -v \(^\(systems/\|share/petsc/datafiles/\)\|/output/\|\.\(png\|pdf\|ps\|ppt\|jpg\)$\)', shell=True)