Use intreehooks for bootstrapping flit_core

Closes gh-298

This is a temporary workaround until a released version of pip supports the
official backend-path mechanism.

doc/history.rst

@@ -8,6 +8,9 @@ Version 2.1
 - Added the :envvar:`FLIT_INSTALL_PYTHON` environment variable (:ghpull:`295`),
   to configure flit to always install into a Python other than the one it's
   running on.
+- ``flit_core`` uses the ``intreehooks`` shim package to load its bootstrapping
+  backend, until a released version of pip supports the standard
+  ``backend-path`` mechanism.
 
 Version 2.0
 -----------

flit_core/pyproject.toml

@@ -1,4 +1,9 @@
 [build-system]
-requires = []
+# Using intreehooks is a workaround until pip supports backend-path;
+# once it does this will be converted to the standard mechanism.
+requires = ["intreehooks"]
+build-backend = "intreehooks:loader"
+
+[tool.intreehooks]
 build-backend = "flit_core.build_thyself"
-backend-path = "."
+#backend-path = "."