Add PEP 517 / PEP 518 metadata

This moves the build-time dependency on incremental to a declarative
file. See [1] for background.

[1]: https://twistedmatrix.com/trac/ticket/9457

Author: twm

pyproject.toml

@@ -1,3 +1,11 @@
+[build-system]
+requires = [
+    "setuptools >= 35.0.2",
+    "wheel >= 0.29.0",
+    "incremental >= 21.3.0",
+]
+build-backend = "setuptools.build_meta"
+
 [tool.towncrier]
 package = "treq"
 package_dir = "src"

tox.ini

@@ -4,6 +4,7 @@ envlist =
     {pypy3,py36,py37,py38,py39}-twisted_latest,
     {pypy3,py36,py37,py38,py39}-twisted_trunk,
     towncrier, twine, check-manifest, flake8, docs
+isolated_build = true
 
 [testenv]
 extras = dev