A few fixes for flit builds

The build backend should be `flit_core`, not `flit`:
https://flit.pypa.io/en/stable/pyproject_toml.html#build-system-section

File `_version.py` should be part of sdist and wheel distributions.

Author: DimitriPapadopoulos

.gitignore

@@ -1,4 +1,4 @@
-/tabulate/version.py
+/tabulate/_version.py
 
 build
 dist

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["flit>=3.12", "flit_scm"]
+requires = ["flit_core>=3.12", "flit_scm"]
 build-backend = "flit_scm:buildapi"
 
 [project]
@@ -34,7 +34,6 @@ tabulate = "tabulate:_main"
 
 [tool.flit.sdist]
 include = ["CHANGELOG", "test/", "tox.ini"]
-exclude = ["tabulate/_version.py"]
 
 [tool.setuptools_scm]
 write_to = "tabulate/_version.py"