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>

git_trace2_event_log_base.py

@@ -76,9 +76,8 @@ class BaseEventLog:
         # Save both our sid component and the complete sid.
         # We use our sid component (self._sid) as the unique filename prefix and
         # the full sid (self._full_sid) in the log itself.
-        self._sid = "repo-%s-P%08x" % (
-            self.start.strftime("%Y%m%dT%H%M%SZ"),
-            os.getpid(),
+        self._sid = (
+            f"repo-{self.start.strftime('%Y%m%dT%H%M%SZ')}-P{os.getpid():08x}"
         )
 
         if add_init_count: