Drop Python 2.7/3.5 testing and support

This drops Python 2.7 testing from bindep and removes trove labels for
Python 2.7 in setup.cfg. We also remove Python 3.5 trove labels as we
stopped testing 3.5 some time ago. Additionally, we stop producing
universal wheels as this is now a python 3 only project.

This has been a long time coming. With Debian Bookworm not shipping a
python2 at all this is a good indication it is time for us to stop using
python2 with Bindep. Those that need Bindep with Python 2 or 3.5 can do
so using older releases. To help make this almost automagic we set
python_requires >= 3.6 which will make pip select an appropriate version
based on the python version being used.

Change-Id: Ief77df0b210c6d236adab2ba6659f181eea17459

.zuul.yaml

@@ -55,7 +55,6 @@
         - bindep-ubuntu-focal
         - build-python-release
         - nox-linters
-        - nox-py27
         - nox-py36:
             nodeset: ubuntu-bionic
         - nox-py310:
@@ -73,7 +72,6 @@
         - bindep-ubuntu-focal
         - build-python-release
         - nox-linters
-        - nox-py27
         - nox-py36:
             nodeset: ubuntu-bionic
         - nox-py310:

setup.cfg

@@ -26,9 +26,7 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: MacOS :: MacOS X
     Operating System :: POSIX :: Linux
-    Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
@@ -38,6 +36,7 @@ classifier =
     Topic :: Software Development :: Testing
     Topic :: System :: Archiving :: Packaging
     Topic :: Utilities
+python_requires = >=3.6
 
 [files]
 packages = bindep
@@ -49,9 +48,6 @@ warnerrors = True
 console_scripts =
     bindep = bindep.__main__:main
 
-[wheel]
-universal = 1
-
 [flake8]
 # E123, E125 skipped as they are invalid PEP-8.
 show-source = True