2.3.1b4 (#1184)

Including changes to mnexec -c to reduce the number of
close() system calls, which may help with performance in
containers on Ubuntu 22.04.

Note that systemd-udevd also seems to be a cause of performance issues
and should probably be disabled while running Mininet.

Adding dbeug logging to test_simpleperf because it is failing
intermittently. (Note with packet loss it can fail, but this
is not how we should be testing it.)

2.3.1b3 (#1181)

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.

2.3.0 (#1043)

- 2.3.0rc2 -> 2.3.0
- update copyright date in LICENSE

2.3.0rc2 (#1042)

Update INSTALL

2.3.0b2 (#1022)

+ text file updates

Mininet 2.3.0b1 (#1019)

Also tweak README.md, INSTALL, and CONTRIBUTORS