Add CI support for ruby 3 and fix broken spec

Spec started failing because a statement which we expected to be a
syntax error is now interpreted as a valid pattern-matching statement.
Swapping the hash-rockets for colons turns this back into a syntax
error.

.circleci/config.yml

@@ -138,6 +138,15 @@ jobs:
       - <<: *bundle_install
       - <<: *install_ubuntu_nano
       - <<: *unit
+  "ruby-3.0":
+    docker:
+      - image: circleci/ruby:3.0
+    working_directory: ~/pry
+    steps:
+      - <<: *repo_restore_cache
+      - <<: *bundle_install
+      - <<: *install_ubuntu_nano
+      - <<: *unit
   "jruby-9.1-jdk":
     docker:
       - image: circleci/jruby:9.1-jdk
@@ -199,6 +208,10 @@ workflows:
           requires:
             - rubocop_lint
             - yard_lint
+      - "ruby-3.0":
+          requires:
+            - rubocop_lint
+            - yard_lint
       - "jruby-9.1-jdk":
           requires:
             - rubocop_lint