setup: Remove attribute 'use_2to3'

REASON:
* This attribute is deprecated since setuptools >= v58.0.2 (2021-09-06).
* 2to3 conversion should not be needed anymore.
  Currently, code should run on python2 and python3 (by using six, etc.).

setup.py

@@ -118,8 +118,8 @@ def find_packages_by_root_package(where):
             "pylint",
         ],
     },
-    # MAYBE-DISABLE: use_2to3
-    use_2to3= bool(python_version >= 3.0),
+    # DISABLED: use_2to3= bool(python_version >= 3.0),
+    # DEPRECATED SINCE: setuptools v58.0.2 (2021-09-06)
     license="BSD",
     classifiers=[
         "Development Status :: 4 - Beta",
@@ -129,12 +129,11 @@ def find_packages_by_root_package(where):
         "Programming Language :: Python :: 2",
         "Programming Language :: Python :: 2.7",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.3",
-        "Programming Language :: Python :: 3.4",
         "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: Implementation :: CPython",
         "Programming Language :: Python :: Implementation :: Jython",
         "Programming Language :: Python :: Implementation :: PyPy",