Update docopt version to 0.7.1

[kojix2]

Hi @brentp,

I encountered an error while trying to compile mosdepth on an Apple M2 macOS. It appears that the issue can be resolved by updating docopt.

mosdepth on master is 📦 v0.3.8 via 👑 v2.0.8 
❯ nimble build -Y mosdepth.nimble
  Verifying dependencies for [email protected]
     Info:  Dependency on hts@>= 0.3.22 already satisfied
  Verifying dependencies for [email protected]
     Info:  Dependency on [email protected] already satisfied
  Verifying dependencies for [email protected]
     Info:  Dependency on regex@>= 0.11.1 already satisfied
  Verifying dependencies for [email protected]
     Info:  Dependency on unicodedb@>= 0.7.2 already satisfied
  Verifying dependencies for [email protected]
     Info:  Dependency on https://github.com/brentp/d4-nim@>= 0.0.3 already satisfied
  Verifying dependencies for [email protected]
   Building mosdepth/mosdepth using c backend
/Users/kojix2/.nimble/pkgs2/docopt-0.7.0-21150284640b882fa91147181c52da3e5bb44df8/docopt.nim(608, 6) Error: 'docopt' is not GC-safe as it calls 'docopt_exc'
       Tip: 12 messages have been suppressed, use --verbose to show them.
nimble.nim(304)          buildFromDir

    Error:  Build failed for the package: mosdepth

For more details, please see docopt/docopt.nim#57 for more detail.

Thank you.