[BEAM-6593] Python 3 IT on DirectRunner in PreCommit

[markflyhigh]

• Add WordCountIT.test_wordcount_it on DirectRunner in PreCommit suite. The tests are added to gradle :beam-sdks-python-precommit-direct-py3 and can run in parallel with others.
• In cleanPython, delete target directory which's used for tox build.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---	---	---	---	---
Java
Python		---			---	---	---

[markflyhigh]

cc: @tvalentyn @RobbeSneyders

[charlesccychen]

Run Website PreCommit

[markflyhigh]

Run Website PreCommit

[charlesccychen]

Thanks! This LGTM.

[charlesccychen]

Website failures are unrelated and are fixed by #7740.