2.3.1b2 (#1172)

Support for Ubuntu 22.04 and 20.04.

- Update Mininet version
- Install pexpect with pip to support python 3 and 2
- Update code-check workflow
  - refactor to enable testing python 2 on ubuntu 22.04
  - update to actions/checkout@v3 (@v2 is deprecated)

errata:

mountCgroups has not actually been renamed, but has been
changed to use cgroupfs-mount and to return the cgroups version.

README.md

@@ -2,7 +2,7 @@ Mininet: Rapid Prototyping for Software Defined Networks
 ========================================================
 *The best way to emulate almost any network on your laptop!*
 
-Mininet 2.3.1b1
+Mininet 2.3.1b2
 
 [![Build Status][1]](https://github.com/mininet/mininet/actions)
 
@@ -70,7 +70,7 @@ Mininet includes:
 
 ### Python 3 Support
 
-- Mininet 2.3.1b1 supports Python 3 and Python 2!
+- Mininet 2.3.1b2 supports Python 3 and Python 2
 
 - You can install both the Python 3 and Python 2 versions of
 Mininet side by side, but the most recent installation will
@@ -87,10 +87,12 @@ determine which Python version is used by default by `mn`.
 
 ### Other Enhancements and Information
 
-- Support for Ubuntu 20.04 LTS (and 18.04)
+- Support for Ubuntu 22.04 LTS (and 20.04)
 
 - More reliable testing and CI via github actions
 
+- Preliminary support for cgroups v2 (and v1)
+
 - Minor bug fixes (2.3.1)
 
 - Additional information about this release and previous releases