chore: fix build on py36

[wey-gu]

To build nebula-python in py36, we have to call python setup.py install (only impact those who will to build nebula-python from source in py36)

Note, the wheel built now in Nebula-Python could be installed with pip install <file>.whl

dependent on https://github.com/vesoft-inc/nebula-python/pull/307

In summary, now:

• NebulaGraph-Py supports python3.6.2+ to be installed from its package
• in py3.7+, it could be built properly with pdm build or other tools that support pyproject.toml
• in py3.6, if we would like to build from python repo source code, we have to do python3 setup.py install (the only draw-back, which impacted our tests makefile.)

See more from: https://github.com/vesoft-inc/nebula-python/issues/308

What type of PR is this?

• [x] bug
• [ ] feature
• [ ] enhancement