fix: support conn_pool ping before init | add missing dep in py3.6

[wey-gu]

In NebulaGraph repo ci test, ping was done before the connection pool's init, this was broken in recent changes. This change fixed it.

issue 1:

a. connection_pool cannot be ping() without init()

b. dependency issue:

• http2 was added, it relied on httpcore-->anyio-->contextvars(only needed when python < 3.7)
• in setup.py, there were httpx[http2] >= 0.22.0, ensures httpcore-->anyio BUT NOT contextvars

WHY this issue(a, or b) was not found? It could be due to a dirty cache, see issue 2 hided issue 1.

Action: added this dep handling in setup.py

issue 2:

dirty cache in gh runner in /home/vesoft/actions-runner/_work/_temp/_github_home/.local

docker run --rm -e "HOME=/github/home" -v "/home/vesoft/actions-runner/_work/_temp/_github_home":"/github/home" -it vesoft/nebula-dev:centos7

bash-4.2# pip3 freeze | grep nebula
nebula3-python==3.5.1

>>> sys.path
['', '/usr/lib64/python36.zip', '/usr/lib64/python3.6', '/usr/lib64/python3.6/lib-dynload', '/github/home/.local/lib/python3.6/site-packages', '/usr/local/lib/python3.6/site-packages', '/usr/lib64/python3.6/site-packages', '/usr/lib/python3.6/site-packages']
>>> import nebula3
>>> nebula3.__file__
'/github/home/.local/lib/python3.6/site-packages/nebula3/__init__.py'

WHY .local exist unexpected nebula3-python package? It could be caused by attempts of pip3 install --user:

bash-4.2# pip3 install --user nebula2-python
WARNING: Running pip install with root privileges is generally not a good idea. Try `pip3 install --user` instead.
Collecting nebula2-python
  Downloading https://files.pythonhosted.org/packages/49/a8/75bf56857e3d42f33d569b449be3435f82b783a4d57fcaab34349cc029bd/nebula2_python-2.6.1-py3-none-any.whl (301kB)
    100% |████████████████████████████████| 307kB 1.2MB/s
Requirement already satisfied: pytz in /github/home/.local/lib/python3.6/site-packages (from nebula2-python)
Requirement already satisfied: six in /github/home/.local/lib/python3.6/site-packages (from nebula2-python)
Requirement already satisfied: future in /github/home/.local/lib/python3.6/site-packages (from nebula2-python)
Requirement already satisfied: httplib2 in /github/home/.local/lib/python3.6/site-packages (from nebula2-python)
Requirement already satisfied: pyparsing!=3.0.0,!=3.0.1,!=3.0.2,!=3.0.3,<4,>=2.4.2; python_version > "3.0" in /github/home/.local/lib/python3.6/site-packages (from httplib2->nebula2-python)
Installing collected packages: nebula2-python
Successfully installed nebula2-python-2.6.1

bash-4.2# pip3 uninstall nebula2-python -y
Uninstalling nebula2-python-2.6.1:
  Successfully uninstalled nebula2-python-2.6.1

And in vesoft-inc/nebua, it's doing so(install 3.5.1 with pip3 install --user

install-nebula-py: install-deps
    git clone --branch release-3.5 https://github.com/vesoft-inc/nebula-python $(CURR_DIR)/nebula-python
    cd $(CURR_DIR)/nebula-python \
        && pip3 install --user . -i $(PYPI_MIRROR) --upgrade
    rm -rf $(CURR_DIR)/nebula-python

Action: force to remove unexpected nebula3-python packages before installation first, we need to ensure .local is cleaned in runner cache.