chore: Update build system to flit_core (#155)

Hi, I'm opening this pull request as part of a push to modernise how
packages use [Flit](https://flit.pypa.io/en/stable/) as a Python build
backend.

Using `flit_core` as the backend in place of `flit` is [recommended in
the
docs](https://flit.pypa.io/en/stable/pyproject_toml.html#build-system-section),
and will make it faster for tools like pip &
[build](https://build.pypa.io/en/latest/) to build your package from
source, as it has fewer dependencies to install.

Specifying an explicit version range (`>=2,<4`) helps to ensure that
your package can still be readily built from source despite changes in
future major versions of Flit, because it will still use version 3.x.
For instance, a future version is likely to drop support for the
`[tool.flit.metadata]` table, in favour of the now-standardised
`[project]` table for metadata. This is also [in the
docs](https://flit.pypa.io/en/stable/pyproject_toml.html#build-system-section),
along with details of which versions support which features.

Most users probably install your package from a pre-built 'wheel' on
PyPI, so this changes won't affect them at all. But people who install
from a git checkout, for instance, will benefit.

pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
-requires = ["flit"]
-build-backend = "flit.buildapi"
+requires = ["flit_core >=2,<4"]
+build-backend = "flit_core.buildapi"
 
 [project]
 name =  "py-project-toml"