Drop Python2.7 support

Because Python2.7 is last release of Python2.x series and
Python2.7 support stopped in 2020, we need to drop Python2.7
related things. This commit cleanups Python2.7 related contents.

Change-Id: I92eed041d993af924d93373977aef33cda0f7cf8

py27-constraints.txt

@@ -1 +0,0 @@
-stestr==2.6.0  # Apache-2.0

setup.cfg

@@ -15,7 +15,6 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python
-    Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
 
 [files]

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.9.0
-envlist = py3,py27,pep8
+envlist = py3,pep8
 skipsdist = True
 ignore_basepython_conflict=true
 
@@ -27,12 +27,6 @@ commands =
 basepython = python3
 commands = {posargs}
 
-[testenv:py27]
-deps =
-  -c{toxinidir}/py27-constraints.txt
-  -r{toxinidir}/requirements.txt
-  -r{toxinidir}/test-requirements.txt
-
 [testenv:cover]
 basepython = python3
 setenv =