Clean up tox deps env (#185)

* tox deps: set basepython to python3.10

The minimum supported Python version is now python3.10, so we should use
that to generate the constraints file.

* tox deps: use skip_install

We don't need to install the package itself to run pip-compile.

* constraints: regenerate file with tox -e deps

.config/constraints.txt

@@ -1,5 +1,5 @@
 #
-# This file is autogenerated by pip-compile with Python 3.9
+# This file is autogenerated by pip-compile with Python 3.10
 # by the following command:
 #
 #    pip-compile --all-extras --no-annotate --output-file=.config/constraints.txt --strip-extras pyproject.toml

tox.ini

@@ -56,7 +56,8 @@ extras =
 description = Update dependency lock files
 # Force it to use oldest supported version of python or we would lose ability
 # to get pinning correctly.
-basepython = python3.9
+basepython = python3.10
+skip_install = true
 deps =
     pip-tools >= 6.12.3
 setenv =