Make selinux an optional extra (#89)

As our special selinux shim dependency also caused problems for
people that did not really need selinux, we now make it a simple
extra.

Author: ssbarnea

pyproject.toml

@@ -68,10 +68,13 @@ docker = [
 ec2 = []
 gce = []
 podman = [
-    # selinux python module is needed as least by ansible-podman modules
-    # and allows us of isolated (default) virtualenvs. It does not avoid need
-    # to install the system selinux libraries but it will provide a clear
-    # message when user has to do that.
+]
+selinux = [
+    # selinux python module is needed as least by podman and docker on systems
+    # that do have selinux enabled and where code is running inside of an
+    # isolated (default) virtualenv. It does not avoid need to install the
+    # system selinux libraries but it will provide a clear message when user
+    # has to do that.
     'selinux; sys_platform=="linux2"',
     'selinux; sys_platform=="linux"',
 ]