Update python-dateutil 2.6.1 -> 2.7.3

[pacien]

Motivation for this change

Various bugfixes.

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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[xeji]

Please rebase this PR on the staging branch since it causes >22k rebuild.

[FRidh]

Bumps of this package typically cause quite some breakage, so don't merge. This needs more investigation.

[pacien]

The PR has been rebased.

Version 2.7.x (changelog) most notably drops support for Python 2.6 and 3.2, whereas versions 2.7 and 3.6 are used in NixOS. Other changes mostly consist of bug fixes, feature addition and a few deprecation. I hope this doesn't break other packages.

As a side note, updating this library would enable the resolution of some issues in other packages like this one.

[pacien]

Seems like this breaks a test in the vobject package: skarim/vobject#113, skarim/vobject#112

[FRidh]

I've included the changes in #41894.