remove backend_pdf from pep8 test exclude list

Author: megies

lib/matplotlib/tests/test_coding_standards.py

@@ -111,7 +111,6 @@
                       '*/matplotlib/backends/backend_gtkcairo.py',
                       '*/matplotlib/backends/backend_macosx.py',
                       '*/matplotlib/backends/backend_mixed.py',
-                      '*/matplotlib/backends/backend_pdf.py',
                       '*/matplotlib/backends/backend_pgf.py',
                       '*/matplotlib/backends/backend_ps.py',
                       '*/matplotlib/backends/backend_svg.py',