Move error code table generation from Makefile to conf.py.

Author: Nurdok

docs/Makefile

@@ -50,8 +50,6 @@ clean:
 	rm -rf $(BUILDDIR)/*
 
 html:
-	@echo "Building error code table from error registry."
-	python -c "import os, sys; sys.path.insert(0, os.path.join(os.getcwd(), '..')); from pep257 import ErrorRegistry; print(ErrorRegistry.to_rst())" > snippets/error_code_table.rst
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."

docs/conf.py

@@ -265,3 +265,10 @@
 # sphinxcontrib.issuetracker settings
 issuetracker = 'github'
 issuetracker_project = 'GreenSteam/pep257'
+
+def generate_error_code_table():
+    from ..pep257 import ErrorRegistry
+    with open(os.path.join('snippets', 'error_code_table.rst'), 'wt') as outf:
+        outf.write(ErrorRegistry.to_rst())
+
+generate_error_code_table()