Limit enum34 installation via environment markers (#8213)

Fixes #7050 

### Motivation

To prevent installing enum34 on systems where it is not needed, declare the dependency with environment markers. Installing certain versions of enum34 on systems where it is not needed at all, might result in compile time issues. This is the recommended approach discussed here:
python-poetry/poetry#1122

### Modifications

* `setup.py`: use environment markers for enum34 instead of custom logic that is not preserved in pypi metadata for the wheel

setup.py

@@ -22,7 +22,6 @@
 from distutils.util import strtobool
 from os import environ
 import subprocess
-import sys
 
 from distutils.command import build_ext
 
@@ -57,11 +56,6 @@ def get_name():
 print(VERSION)
 print(NAME)
 
-if sys.version_info[0] == 2:
-    PY2 = True
-else:
-    PY2 = False
-
 # This is a workaround to have setuptools to include
 # the already compiled _pulsar.so library
 class my_build_ext(build_ext.build_ext):
@@ -83,17 +77,14 @@ def build_extension(self, ext):
     'protobuf>=3.6.1',
     'six',
     'certifi',
+    'enum34>=1.1.9; python_version < "3.4"',
 
     # functions dependencies
     "apache-bookkeeper-client>=4.9.2",
     "prometheus_client",
     "ratelimit"
 ]
 
-if PY2:
-    # Python 2 compat dependencies
-    dependencies += ['enum34>=1.1.9']
-
 setup(
     name=NAME,
     version=VERSION,