Remove remaining code for Python < 3.6

Support for Python 2.7 and Python 3.5 was already removed by
1d4f5113a3379da2568a36cf6277c8b0f670fdea and now only Python 3.6 or
newer versions are supported.

Change-Id: I976a7f6b8230e25f390b8c358211f3f6d5e13274

bindep/tests/test_depends.py

@@ -20,10 +20,7 @@ import os.path
 import platform
 import subprocess
 from textwrap import dedent
-try:
-    from unittest import mock
-except ImportError:
-    import mock
+from unittest import mock
 
 import distro
 import fixtures

bindep/tests/test_main.py

@@ -19,10 +19,7 @@ import os
 import subprocess
 import sys
 from textwrap import dedent
-try:
-    from unittest import mock
-except ImportError:
-    import mock
+from unittest import mock
 
 from fixtures import FakeLogger
 from fixtures import Fixture

doc/requirements.txt

@@ -1,8 +1,6 @@
 reno>=2.8.0 # Apache-2.0
-sphinx!=1.6.6,!=1.6.7,<2.0.0;python_version=='2.7'  # BSD
-sphinx!=1.6.6,!=1.6.7,!=2.1.0;python_version>='3.4'  # BSD
+sphinx!=1.6.6,!=1.6.7,!=2.1.0  # BSD
 sphinxcontrib-programoutput  # BSD license
 
 # needed because we use autodoc to document our tests in the dev docs
 fixtures>=0.3.12
-mock>=2.0  # BSD

requirements.txt

@@ -1,6 +1,4 @@
-distro<1.7.0 ; python_version < '3.6'
-distro>=1.7.0 ; python_version >= '3.6'
+distro>=1.7.0
 pbr>=2.0.0  # Apache-2.0
 Parsley
-packaging ; python_version >= '3.6'
-packaging<21.0 ; python_version < '3.6'
+packaging

test-requirements.txt

@@ -1,6 +1,5 @@
 coverage>=3.6
 fixtures>=0.3.12
-mock>=2.0;python_version<='3.3'  # BSD
 python-subunit
 stestr>=1.0.0 # Apache-2.0
 testtools>=0.9.27