Use an environment marker to install future only on Python 2

An environment marker is already used for `enum34` so why do it
differently for `future`?
https://peps.python.org/pep-0508/#environment-markers

ref #343

Author: michael-k

setup.py

@@ -1,5 +1,3 @@
-import sys
-
 from setuptools import setup, find_packages
 from os import path
 from aws_xray_sdk.version import VERSION
@@ -14,15 +12,6 @@
 
 long_description = read_md(path.join(CURRENT_DIR, 'README.md'))
 
-INSTALL_REQUIRED_DEPS = [
-    'enum34;python_version<"3.4"',
-    'wrapt',
-    'botocore>=1.11.3',
-]
-
-if sys.version_info[0] == 2:
-    INSTALL_REQUIRED_DEPS.append("future")
-
 setup(
     name='aws-xray-sdk',
     version=VERSION,
@@ -55,7 +44,12 @@
         'Programming Language :: Python :: 3.9',
     ],
 
-    install_requires=INSTALL_REQUIRED_DEPS,
+    install_requires=[
+        'enum34;python_version<"3.4"',
+        'wrapt',
+        'future;python_version<"3"',
+        'botocore>=1.11.3',
+    ],
 
     keywords='aws xray sdk',