Fix misused pyproject.toml

pyproject.toml

 [build-system]
+requires = ["flit_core >=2,<4"]
+build-backend = "flit_core.buildapi"
+
+[tool.flit.metadata]
+module = "nbcourse"
+author = "Matthieu Boileau"
+author-email = "matthieu.boileau@math.unistra.fr"
+home-page = "https://gitlab.math.unistra.fr/boileau/nbcourse"
+classifiers = ["License :: OSI Approved :: MIT License"]
+description-file = "README.md"
 requires = [
-    "flit_core >=2,<4",
     "nbconvert >= 6.0",
     "bs4",
     "IPython",
@@ -13,15 +22,6 @@ requires = [
     "doit >= 0.32.0",
     "python-frontmatter",
 ]
-build-backend = "flit_core.buildapi"
-
-[tool.flit.metadata]
-module = "nbcourse"
-author = "Matthieu Boileau"
-author-email = "matthieu.boileau@math.unistra.fr"
-home-page = "https://gitlab.math.unistra.fr/boileau/nbcourse"
-classifiers = ["License :: OSI Approved :: MIT License"]
-description-file = "README.md"
 
 [tool.flit.metadata.requires-extra]
 test = ["pytest"]