build: use pyproject.toml to comply with pep517

Also: remove --no-index for installing ourselves.

PEP-517 compliance means we need to use PyPI to get setuptools, so we
can't use --no-index anymore.  I think this was added just to speed up
the initial installation of the virtualenv used in these tests.

pyproject.toml

@@ -0,0 +1,6 @@
+# Licensed under the Apache License: http://www.apache.org/licenses/LICENSE-2.0
+# For details: https://github.com/nedbat/coveragepy/blob/master/NOTICE.txt
+
+[build-system]
+requires = ['setuptools', 'wheel']
+build-backend = 'setuptools.build_meta'

tests/test_venv.py

@@ -139,7 +139,7 @@ def testp():
 
         # Install everything.
         run_in_venv(
-            "python -m pip install --no-index " +
+            "python -m pip install " +
             "./third_pkg " +
             "-e ./another_pkg " +
             "-e ./bug888/app -e ./bug888/plugin " +