pep8_tap: fix counter

In 1f796f06 I broke the issue counter. This fixes the breakage.
pep8_tap: Fix crash with invalid encoded source data
pep8_tap: ignore non-Python files
tap_tool_dispatcher: run all tools in parallel

Running the different *_tap tools in parallel allows for significant
speedups on our build server; a job with ~440 files took 1min35s
previously, and now takes < 30s to complete.