SPARK-3331 [BUILD] PEP8 tests fail because they check unzipped py4j code

PEP8 tests run on files under "./python", but unzipped py4j code is found at "./python/build/py4j". Py4J code fails style checks and can fail ./dev/run-tests if this code is present locally.

Author: Sean Owen <sowen@cloudera.com>

Closes #2222 from srowen/SPARK-3331 and squashes the following commits:

34711ec [Sean Owen] Restrict lint check to pyspark/, since the local directory can contain unzipped py4j code in build/py4j