Make CI fail if code isn't formatted properly

To test this, I improperly formatted `tasks.py`. I expect CI to fail.

For:
#60

.github/workflows/check.yml

@@ -24,7 +24,7 @@ jobs:
       - name: seed the database
         run: pipenv run invoke dbseed
       - name: format
-        run: pipenv run invoke fmt
+        run: pipenv run invoke fmtcheck
       - name: lint
         run: pipenv run invoke lint
       - name: check types

tasks.py

@@ -32,6 +32,11 @@ def fmt(c):
     c.run("mdformat README.md")
     c.run("mdformat doc")
 
+@task
+def fmtcheck(c):
+    """Checks if the code is formatted properly."""
+    c.run("black --check .")
+
 
 @task
 def lint(c):