build: Remove transient OUTPUT directory after install

* While doing multiple rebuilds it is advisable to also clean up the
  state of the Git submodules with 'git clean -f', removing the OUTPUT
  directory at src/fastjet/_fastjet_core/ and the top level build directory
  is sufficient to do local development rebuilds --- i.e., the command
  'python -m pip install --upgrade .' can be run twice in a row without error.

setup.py

@@ -29,6 +29,13 @@
 PYTHON = DIR / "src/fastjet"
 OUTPUT = PYTHON / "_fastjet_core"
 
+
+# Clean up transient directories to allow for rebuilds during development
+if (DIR / "build").exists():
+    shutil.rmtree(DIR / "build")
+if OUTPUT.exists():
+    shutil.rmtree(OUTPUT)
+
 LIBS = [
     "fastjet",
     "fastjettools",