setup: move away from legacy build backend (#4790)

The `pyproject.toml` was originally added in ca75832 
without specifying a backend, which implicitly defaults to the legacy `setuptools.build_meta:__legacy__` one. 
This choice was made explicit in a2bebb4 

This can lead to issues when using a *system* version of setuptools
< 40.8.0, see [1].
We believe there is no good reason for sticking with the legacy build system.

I've tested that the `reentry_register` hook still works with the new
build backend.

[1] pypa/setuptools#1694 (comment)

pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
 requires = ["setuptools>=40.8.0", "wheel", "reentry~=1.3", "fastentrypoints~=0.12"]
-build-backend = "setuptools.build_meta:__legacy__"
+build-backend = "setuptools.build_meta"
 
 [tool.pylint.master]
 load-plugins = "pylint_django"

utils/dependency_management.py

@@ -191,7 +191,7 @@ def update_pyproject_toml():
         'requires': ['setuptools>=40.8.0', 'wheel',
                      str(reentry_requirement), 'fastentrypoints~=0.12'],
         'build-backend':
-        'setuptools.build_meta:__legacy__',
+        'setuptools.build_meta',
     })
 
     # write the new file