run unit tests on pyproject.toml changes

The unit tests workflow builds and installs the package before running
tests. So it's useful to trigger the workflow on pyproject.toml changes,
similar to setup.py.

Author: arcctgx

.github/workflows/unit-tests.yml

@@ -10,6 +10,7 @@ on:
       - tests/**
       - setup.py
       - requirements.txt
+      - pyproject.toml
   pull_request:
     branches:
       - master
@@ -19,6 +20,7 @@ on:
       - tests/**
       - setup.py
       - requirements.txt
+      - pyproject.toml
 
 jobs:
   unit-tests: