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>

ssh.py

@@ -165,7 +165,7 @@ class ProxyManager:
         # Check to see whether we already think that the master is running; if
         # we think it's already running, return right away.
         if port is not None:
-            key = "%s:%s" % (host, port)
+            key = f"{host}:{port}"
         else:
             key = host