build: resolve a DeprecationWarning by adding a pyproject.toml (#139)

This resolves the following warning
when running `pip install -e.`:

```
DEPRECATION: Legacy editable install of scriv==1.5.1
from file:///path/to/scriv (setup.py develop)
is deprecated. pip 25.1 will enforce this behaviour
change. A possible replacement is to add a
pyproject.toml or enable --use-pep517, and use
setuptools >= 64. If the resulting installation is
not behaving as expected, try using --config-settings
editable_mode=compat. Please consult the setuptools
documentation for more information. Discussion can be
found at pypa/pip#11457
```

It appears that scriv can still be packaged
by running `python -m build` after this change.

Note that pytest must now be explicitly configured
to load its configuration from `setup.cfg`.
Otherwise, it will detect `pyproject.toml`
and attempt to import tomli, which is uninstalled
when the test suite is running the `-no_extras` factor.

Closes #117

Author: kurtmckee

pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.build_meta"

tox.ini

@@ -28,7 +28,7 @@ setenv =
     no_extras: SCRIV_TEST_NO_EXTRAS=1
 commands =
     no_extras: python -m pip uninstall -q -y tomli
-    coverage run -p -m pytest -Wd {posargs}
+    coverage run -p -m pytest -Wd -c setup.cfg {posargs}
 
 [testenv:deps]
 allowlist_externals =