delete everything related to python2

We are dropping support for python 2, so we are dropping the
verification of which python version is currently running and we also stop
checking for python2 specific packages.
We also drop the encoding definition since python3 uses utf-8 as
default.
The helper open_func function is also substituted by open.

pipreqs/pipreqs.py

@@ -1,5 +1,4 @@
 #!/usr/bin/env python
-# -*- coding: utf-8 -*-
 """pipreqs - Generate pip requirements.txt file based on imports
 
 Usage:
@@ -37,13 +36,11 @@
                           <gt>     | e.g. Flask>=1.1.2
                           <no-pin> | e.g. Flask
 """
-from __future__ import print_function, absolute_import
 from contextlib import contextmanager
 import os
 import sys
 import re
 import logging
-import codecs
 import ast
 import traceback
 from docopt import docopt
@@ -58,14 +55,6 @@
     re.compile(r'^from ((?!\.+).*?) import (?:.*)$')
 ]
 
-if sys.version_info[0] > 2:
-    open_func = open
-    py2 = False
-else:
-    open_func = codecs.open
-    py2 = True
-    py2_exclude = ["concurrent", "concurrent.futures"]
-
 
 @contextmanager
 def _open(filename=None, mode='r'):
@@ -122,7 +111,7 @@ def get_all_imports(
         candidates += [os.path.splitext(fn)[0] for fn in files]
         for file_name in files:
             file_name = os.path.join(root, file_name)
-            with open_func(file_name, "r", encoding=encoding) as f:
+            with open(file_name, "r", encoding=encoding) as f:
                 contents = f.read()
             try:
                 tree = ast.parse(contents)
@@ -157,7 +146,6 @@ def get_all_imports(
     with open(join("stdlib"), "r") as f:
         data = {x.strip() for x in f}
 
-    data = {x for x in data if x not in py2_exclude} if py2 else data
     return list(packages - data)
 
 
@@ -214,7 +202,7 @@ def get_locally_installed_packages(encoding=None):
             for item in files:
                 if "top_level" in item:
                     item = os.path.join(root, item)
-                    with open_func(item, "r", encoding=encoding) as f:
+                    with open(item, "r", encoding=encoding) as f:
                         package = root.split(os.sep)[-1].split("-")
                         try:
                             package_import = f.read().strip().split("\n")
@@ -309,7 +297,7 @@ def parse_requirements(file_):
     delim = ["<", ">", "=", "!", "~"]
 
     try:
-        f = open_func(file_, "r")
+        f = open(file_, "r")
     except OSError:
         logging.error("Failed on file: {}".format(file_))
         raise
@@ -381,7 +369,7 @@ def clean(file_, imports):
     to_write = []
 
     try:
-        f = open_func(file_, "r+")
+        f = open(file_, "r+")
     except OSError:
         logging.error("Failed on file: {}".format(file_))
         raise