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

diff --git a/setup.py b/setup.py
index 3796dc696..1368043ca 100644
--- a/setup.py
+++ b/setup.py
@@ -64,6 +64,7 @@ setup(
               'kubernetes.dynamic', 'kubernetes.leaderelection'],
     include_package_data=True,
     long_description="Python client for kubernetes http://kubernetes.io/",
+    python_requires='>=3.6',
     classifiers=[
         "Development Status :: %s" % DEVELOPMENT_STATUS,
         "Topic :: Utilities",
['setup.py']