Add tox for testing with different Python versions.

Author: adamheins

README.md

@@ -136,5 +136,8 @@ problem. Currently known issues:
   however.
 * Ghost objects sometimes flicker (spooky, but undesirable).
 
+## Development
+* Run `tox` to run the tests.
+
 ## License
 [MIT](https://github.com/adamheins/pyb_utils/blob/main/LICENSE)

tox.ini

@@ -0,0 +1,10 @@
+[tox]
+isolated_build = True
+skipsdist = True
+envlist = py{38,39,310,311}
+
+[testenv]
+allowlist_externals = poetry
+commands =
+  poetry install -v
+  poetry run pytest tests/ {posargs}