Reset Sphinx env in lineref code blocks to update references

Running new builds without deleting the `_build` directory first
used to cause warnings "Line reference labels should be unique".
This commit deletes the build environment variable at the start of
the build so that the new build updates the line references
correctly and does not emit warnings.

directives/codeblock_lineref.py

@@ -158,10 +158,18 @@ def lineref_role(name, rawtext, text, lineno, inliner, options={}, content=[]):
          return [linknode], []
 
 
+def _reset_env_code_line_labels(app):
+    env = app.builder.env
+    if hasattr(env, 'code_line_labels'):
+        del env.code_line_labels
+
+
 def setup(app):
     app.add_directive('lineref-code-block', LineRefCodeBlock)
     app.add_node(codeblock_lineref,
                  html=(visit_codeblock_lineref_node, depart_codeblock_lineref_node),
                  latex=(visit_codeblock_lineref_node, depart_codeblock_lineref_node),
                  text=(visit_codeblock_lineref_node, depart_codeblock_lineref_node))
     app.add_role('lref', lineref_role)
+
+    app.connect("builder-inited", _reset_env_code_line_labels)