Fixed enum package not found (apache#6401)

Fixes apache#6400

### Motivation
This problem is blocking the current test. 1.1.8 version of `enum34` seems to have some problems, and the problem reproduces:

Use pulsar latest code:
```
cd pulsar
mvn clean install -DskipTests
dokcer pull apachepulsar/pulsar-build:ubuntu-16.04
docker run -it -v $PWD:/pulsar --name pulsar apachepulsar/pulsar-build:ubuntu-16.04 /bin/bash
docker exec -it pulsar /bin/bash
cmake .
make -j4 && make install 
cd python
python setup.py bdist_wheel
pip install dist/pulsar_client-*-linux_x86_64.whl
```
`pip show enum34`
```
Name: enum34
Version: 1.1.8
Summary: Python 3.4 Enum backported to 3.3, 3.2, 3.1, 2.7, 2.6, 2.5, and 2.4
Home-page: https://bitbucket.org/stoneleaf/enum34
Author: Ethan Furman
Author-email: ethan@stoneleaf.us
License: BSD License
Location: /usr/local/lib/python2.7/dist-packages
Requires:
Required-by: pulsar-client, grpcio
```

```
root@55e06c5c770f:/pulsar/pulsar-client-cpp/python# python
Python 2.7.12 (default, Oct  8 2019, 14:14:10)
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from enum import Enum, EnumMeta
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named enum
>>> exit()
```

There is no problem with using 1.1.9 in the test.

### Modifications

* Upgrade enum34 from 1.1.8 to 1.1.9

### Verifying this change

local test pass

pulsar-client-cpp/python/setup.py

@@ -92,7 +92,7 @@ def build_extension(self, ext):
 
 if PY2:
     # Python 2 compat dependencies
-    dependencies += ['enum34']
+    dependencies += ['enum34>=1.1.9']
 
 setup(
     name=NAME,