cleanup: Update codebase to expect Python 3.6

- Bump minimum version to Python 3.6.
- Use f-strings in a lot of places.

Change-Id: I2aa70197230fcec2eff8e7c8eb754f20c08075bb
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/389034
Tested-by: Jason R. Coombs <jaraco@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Jason R. Coombs <jaraco@google.com>

hooks.py

@@ -180,7 +180,7 @@ class RepoHook:
                 abort_if_user_denies was passed to the consturctor.
         """
         hooks_config = self._hooks_project.config
-        git_approval_key = "repo.hooks.%s.%s" % (self._hook_type, subkey)
+        git_approval_key = f"repo.hooks.{self._hook_type}.{subkey}"
 
         # Get the last value that the user approved for this hook; may be None.
         old_val = hooks_config.GetString(git_approval_key)
@@ -193,7 +193,7 @@ class RepoHook:
             else:
                 # Give the user a reason why we're prompting, since they last
                 # told us to "never ask again".
-                prompt = "WARNING: %s\n\n" % (changed_prompt,)
+                prompt = f"WARNING: {changed_prompt}\n\n"
         else:
             prompt = ""
 
@@ -241,9 +241,8 @@ class RepoHook:
         return self._CheckForHookApprovalHelper(
             "approvedmanifest",
             self._manifest_url,
-            "Run hook scripts from %s" % (self._manifest_url,),
-            "Manifest URL has changed since %s was allowed."
-            % (self._hook_type,),
+            f"Run hook scripts from {self._manifest_url}",
+            f"Manifest URL has changed since {self._hook_type} was allowed.",
         )
 
     def _CheckForHookApprovalHash(self):
@@ -262,7 +261,7 @@ class RepoHook:
             "approvedhash",
             self._GetHash(),
             prompt % (self._GetMustVerb(), self._script_fullpath),
-            "Scripts have changed since %s was allowed." % (self._hook_type,),
+            f"Scripts have changed since {self._hook_type} was allowed.",
         )
 
     @staticmethod