Python2 support dropped but installation works

**What happened (please include outputs or screenshots)**:

I have accidentally installed the git-Master-Branch with Python2 with these commands:

```

git clone --recursive https://github.com/kubernetes-client/python.git

cd python

python setup.py install

```

Everything works fine, but Python2 support is dropped (https://github.com/kubernetes-client/python/pull/1468)



**What you expected to happen**:

I expect an error Message like:

```

  File "setup.py", line 30, in <module>

    raise RuntimeError("kubernetes-client " + CLIENT_VERSION + " requires Python 3.6+")

RuntimeError: kubernetes-client 18.0.0-snapshot requires Python 3.6+

```





**How to reproduce it (as minimally and precisely as possible)**:

See in "What happened". 

You may have to use `python2 setup.py install` if `python --v` is not 2



**Anything else we need to know?**:

A possible fix in setup.py would be

```

import sys



from setuptools import setup



# Do not edit these constants. They will be updated automatically

# by scripts/update-client.sh.

CLIENT_VERSION = "18.0.0-snapshot"

PACKAGE_NAME = "kubernetes"

DEVELOPMENT_STATUS = "3 - Alpha"



# To install the library, run the following

#

# python setup.py install

#

# prerequisite: setuptools

# http://pypi.python.org/pypi/setuptools



if sys.version_info < (3, 6):

    raise RuntimeError("kubernetesclient " + CLIENT_VERSION + " requires Python 3.6+")

```

There is also some Python2 compatibility code in the python/ and python-base repo

For example:

- https://github.com/kubernetes-client/python/blob/master/kubernetes/client/apis/__init__.py#L1

- https://github.com/kubernetes-client/python-base/blob/0d4f822f4f654361764a6f8dcf7ca9bf5bbce3f9/leaderelection/leaderelection.py#L23

- https://github.com/kubernetes-client/python-base/blob/master/watch/watch.py#L32

- https://github.com/kubernetes-client/python-base/blob/b4d3aad42dc23e7a6c0e5c032691f8dc385a786c/dynamic/client.py#L60

Should that also be removed?



**Environment**:

- Kubernetes version (`kubectl version`):

v1.19.0

- OS (e.g., MacOS 10.13.6):

Fedora 29 (Workstation Edition)

- Python version (`python --version`)

Python 2.7.17

- Python client version (`pip list | grep kubernetes`)

18.0.0-snapshot

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