Add python3 support to tryserver recipe module

Bug: 1227140
Change-Id: I54b9fb426e0471543be6c7801f5d5f76ff21c6f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3010715Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

recipes/README.recipes.md

@@ -879,7 +879,7 @@ Args:
 
 Returned paths will be relative to to patch_root.
 
-— **def [get\_footer](/recipes/recipe_modules/tryserver/api.py#333)(self, tag, patch_text=None):**
+— **def [get\_footer](/recipes/recipe_modules/tryserver/api.py#334)(self, tag, patch_text=None):**
 
 Gets a specific tag from a CL description
 
@@ -902,9 +902,9 @@ Returns true iff the properties exist to match a Gerrit issue.
 
 Returns true iff we have a change to check out.
 
-— **def [normalize\_footer\_name](/recipes/recipe_modules/tryserver/api.py#341)(self, footer):**
+— **def [normalize\_footer\_name](/recipes/recipe_modules/tryserver/api.py#342)(self, footer):**
 
-— **def [set\_change](/recipes/recipe_modules/tryserver/api.py#344)(self, change):**
+— **def [set\_change](/recipes/recipe_modules/tryserver/api.py#345)(self, change):**
 
 Set the gerrit change for this module.
 

recipes/recipe_modules/tryserver/api.py

@@ -321,7 +321,8 @@ class TryserverApi(recipe_api.RecipeApi):
       self._ensure_gerrit_commit_message()
       self._change_footers = self._get_footer_step(self._gerrit_commit_message)
       return self._change_footers
-    raise "No patch text or associated changelist, cannot get footers"  #pragma: nocover
+    raise Exception(
+        'No patch text or associated changelist, cannot get footers')  #pragma: nocover
 
   def _get_footer_step(self, patch_text):
     result = self.m.python(