Relax doctest boundaries. (#32)

Co-authored-by: Andreas Ländle <[email protected]>

Author: alaendle

co-log-core.cabal

@@ -101,5 +101,5 @@ test-suite doctest
   type:                 exitcode-stdio-1.0
   hs-source-dirs:       test
   main-is:              Doctests.hs
-  build-depends:        doctest >= 0.16.0 && < 0.21
+  build-depends:        doctest >= 0.16.0 && < 0.22
                       , Glob ^>= 0.10.0