Motivation for this change

I'm not sure what to do about this one - alongside the up-to-date version of pythonPackages.cmd2 we also have pythonPackages.cmd2_8 (0.8.0). It seems we've been keeping this for python 2.7 (see commit message 65b15ba), and indeed cmd2 actually supports py2.7 right up to 0.8.6 (https://github.com/python-cmd2/cmd2/blob/master/CHANGELOG.md#085-april-15-2018).

But unfortunately the recent bump of pyparsing from 2.3.0 to 2.3.1 (c78bf91) broke even 0.8.6's tests (probably because of https://github.com/pyparsing/pyparsing/blob/2809cc975277e5b8c9160e1bac903d414eab305d/CHANGES#L100). So we'd either have to disable more tests than I'd like or maintain an old version of pyparsing too. And seeing as there don't actually seem to be any in-tree reverse dependencies of cmd2_8, it seems most sensible to just remove it - we can't go on offering a whole parallel package selection for 2.7...

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.