[FIX][report_xls] Fix regression introduced in 65223f4.

Variables are now at module level because they have to be mocked when there is
an `ImportError`, so we got an `AttributeError` when trying to use them.

report_xls/report_xls.py

@@ -208,7 +208,7 @@ class report_xls(report_sxw):
                         c.append({'formula': s[5]})
                     else:
                         c.append({
-                            'write_cell_func': report_xls.xls_types[c[3]]})
+                            'write_cell_func': xls_types[c[3]]})
                     # Set custom cell style
                     if s_len > 6 and s[6] is not None:
                         c.append(s[6])
@@ -237,7 +237,7 @@ class report_xls(report_sxw):
             style = spec[6] and spec[6] or row_style
             if not data:
                 # if no data, use default values
-                data = report_xls.xls_types_default[spec[3]]
+                data = xls_types_default[spec[3]]
             if size != 1:
                 if formula:
                     ws.write_merge(