Manually specify pep8 version in tox.ini
Manually specify pep8 version in tox.ini

Specify 1.5.7 for now, because that is the latest version that does
not complain about the current state of the PKB code.

meteorfox/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker