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_command.py

@@ -196,12 +196,10 @@ class UserAgent:
     def git(self):
         """The UA when running git."""
         if self._git_ua is None:
-            self._git_ua = "git/%s (%s) git-repo/%s" % (
-                git.version_tuple().full,
-                self.os,
-                RepoSourceVersion(),
+            self._git_ua = (
+                f"git/{git.version_tuple().full} ({self.os}) "
+                f"git-repo/{RepoSourceVersion()}"
             )
-
         return self._git_ua
 
 
@@ -216,7 +214,7 @@ def git_require(min_version, fail=False, msg=""):
         need = ".".join(map(str, min_version))
         if msg:
             msg = " for " + msg
-        error_msg = "fatal: git %s or later required%s" % (need, msg)
+        error_msg = f"fatal: git {need} or later required{msg}"
         logger.error(error_msg)
         raise GitRequireError(error_msg)
     return False
@@ -243,7 +241,7 @@ def _build_env(
         env["GIT_SSH"] = ssh_proxy.proxy
         env["GIT_SSH_VARIANT"] = "ssh"
     if "http_proxy" in env and "darwin" == sys.platform:
-        s = "'http.proxy=%s'" % (env["http_proxy"],)
+        s = f"'http.proxy={env['http_proxy']}'"
         p = env.get("GIT_CONFIG_PARAMETERS")
         if p is not None:
             s = p + " " + s
@@ -468,7 +466,7 @@ class GitCommand:
                 )
             except Exception as e:
                 raise GitPopenCommandError(
-                    message="%s: %s" % (command[1], e),
+                    message=f"{command[1]}: {e}",
                     project=self.project.name if self.project else None,
                     command_args=self.cmdv,
                 )