python3: disable Python3 bytecode generation by default

Same as python.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
python3: add $(PKG_INSTALL_DIR)/$(PYTHON3_PKG_DIR) to PYTHON3PATH

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Merge pull request #620 from commodo/python3-update

Python3 update