Update build-system entry in pyproject.toml

Update the `build-backend` attribute, in accordance with the Python
Poetry docs [1]. Also, bump the minimum required poetry-core version to
1.2.0, since this is the version that introduced the Poetry dependency
groups [2], i.e., the [tool.poetry.group] sections in pyproject.toml.

[1]: https://python-poetry.org/docs/pyproject/#poetry-and-pep-517
[2]: https://python-poetry.org/docs/managing-dependencies/#dependency-groups

pyproject.toml

@@ -51,5 +51,5 @@ skip = [".gitignore"]
 follow_links = false
 
 [build-system]
-requires = ["poetry>=1.1.4"]
-build-backend = "poetry.masonry.api"
+requires = ["poetry-core>=1.2.0"]
+build-backend = "poetry.core.masonry.api"