Bump Cabal version to 2.0

This will allow to use multiple internal libraries, which can be used to add
tests for other executables than the main one.

Unfortunately, cabal-doctest is currently not supported in combination with
internal libraries and so I had to temporarily disable doctests.

copar.cabal

@@ -8,12 +8,8 @@ author:              Hans-Peter Deifel
 maintainer:          hans-peter.deifel@fau.de
 -- copyright:
 -- category:
-build-type:          Custom
-cabal-version:       >=1.24
-
-custom-setup
-  setup-depends:
-    base, Cabal, cabal-doctest >= 1.0.2 && <1.1
+build-type:          Simple
+cabal-version:       >=2.0
 
 flag release
   description:         Disable sanity checks for more speed and less safety
@@ -188,14 +184,19 @@ test-suite spec
                      , copar
   build-tool-depends: hspec-discover:hspec-discover ==2.*
 
-test-suite doctests
-  type:                exitcode-stdio-1.0
-  hs-source-dirs:      tests
-  main-is:             DocTests.hs
-  default-language:    Haskell2010
-  build-depends:       base >= 4.11
-                     , copar
-                     , doctest
+
+-- FIXME: doctests are currently disabled because cabal-doctest requires a
+-- custom build-type and cabal 2.4 doesn't support custom bulid types together
+-- with multiple internal libraries.
+--
+-- test-suite doctests
+--   type:                exitcode-stdio-1.0
+--   hs-source-dirs:      tests
+--   main-is:             DocTests.hs
+--   default-language:    Haskell2010
+--   build-depends:       base >= 4.11
+--                      , copar
+--                      , doctest
 
 test-suite examples
   type:                exitcode-stdio-1.0