Fix: Clean up generated parser files

The setup.py file overrides the build step to generate parsetab.py and
lextab.py, but does not clean them up. This adds an override to the
clean step to remove them again.

Note that this uses the distutils version of the command as a basis,
since (unlike the build_py step) setuptools does not provide its own
version of it but just uses the distutils version.

setup.py

@@ -2,6 +2,9 @@
 
 from setuptools import Distribution, Extension, find_packages, setup
 
+import contextlib
+import os
+
 try:
     # Update the version by querying git if possible.
     from nml import version_update
@@ -16,6 +19,7 @@
 
 default_dist = Distribution()
 default_build_py = default_dist.get_command_class('build_py')
+default_clean = default_dist.get_command_class('clean')
 
 
 class NMLBuildPy(default_build_py):
@@ -28,6 +32,18 @@ def run(self):
         super().run()
 
 
+class NMLClean(default_clean):
+    def run(self):
+        # Remove python files generated by custom build command above
+        with contextlib.suppress(FileNotFoundError):
+            os.remove('nml/generated/parsetab.py')
+        with contextlib.suppress(FileNotFoundError):
+            os.remove('nml/generated/lextab.py')
+
+        # Then continue with the normal setuptools build.
+        super().run()
+
+
 setup(
     name="nml",
     version=NML_VERSION,
@@ -62,5 +78,5 @@ def run(self):
         "Pillow>=3.4",
         "ply",
     ],
-    cmdclass={"build_py": NMLBuildPy},
+    cmdclass={"build_py": NMLBuildPy, 'clean': NMLClean},
 )