Motivation for this change

Update to latest release (https://github.com/dbcli/mycli/blob/master/changelog.md#1211).

Currently the setup.py restricts prompt_toolkit to < 3.0. Upstream has adjusted the supported version range to >=3.0.0,<4.0.0 a few days after the 1.21.1 release.

I did try the tool with the current prompt_toolkit version in nixpkgs and noticed no obvious problems. The postPatch should be removed with the next upstream release.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)

/nix/store/6xxp4vd31cpsvhkjij7n3knc52xfihd8-mycli-1.20.1	  172056576
/nix/store/nndmxssy3xljzxhfl8cjvqrlc2p4hrcs-mycli-1.21.1	  172307888

• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.