Revert "support ghc 9.0"

This reverts commit 2214aaf.

GHC 9 support will have to wait on the 'hashable' package

haskell-unordered-containers/hashable#193

.github/workflows/haskell.yml

@@ -10,7 +10,6 @@ jobs:
     strategy:
       matrix:
         ghc:
-          - '9.0.1'
           - '8.10.3'
           - '8.8.4'
           - '8.6.5'

invert.cabal

@@ -25,7 +25,7 @@ description:
     well choose only to read and borrow its techniques.
 
 build-type: Simple
-tested-with: GHC==9.0.1, GHC==8.10.3, GHC==8.8.4, GHC==8.6.5
+tested-with: GHC==8.10.3, GHC==8.8.4, GHC==8.6.5
 
 license: Apache-2.0
 license-file: license.txt
@@ -41,7 +41,7 @@ library
       , NamedFieldPuns
       , ExistentialQuantification
     build-depends:
-        base ^>= 4.12 || ^>= 4.13 || ^>= 4.14 || ^>= 4.15
+        base ^>= 4.12 || ^>= 4.13 || ^>= 4.14
       , containers ^>=0.6
       , hashable ^>= 1.3
       , unordered-containers ^>= 0.2
@@ -55,7 +55,7 @@ test-suite billing-codes-example
     main-is: billing-codes.hs
     hs-source-dirs: examples
     build-depends:
-        base ^>= 4.12 || ^>= 4.13 || ^>= 4.14 || ^>= 4.15
+        base ^>= 4.12 || ^>= 4.13 || ^>= 4.14
       , invert
 
 benchmark benchmarks
@@ -66,6 +66,6 @@ benchmark benchmarks
     main-is: bench.hs
     ghc-options: -O2 -Wall
     Build-Depends:
-        base ^>= 4.12 || ^>= 4.13 || ^>= 4.14 || ^>= 4.15
+        base ^>= 4.12 || ^>= 4.13 || ^>= 4.14
       , criterion ^>= 1.5
       , invert