pw_presubmit, pw_watch: pylint fixes

Change-Id: I78e9db5f90925155043d86e53e00e3632ef8edf7
diff --git a/pw_presubmit/py/pw_presubmit/tools.py b/pw_presubmit/py/pw_presubmit/tools.py
index 691e9f7..e36efac 100644
--- a/pw_presubmit/py/pw_presubmit/tools.py
+++ b/pw_presubmit/py/pw_presubmit/tools.py
@@ -363,7 +363,7 @@
         output_directory = self._output_directory.joinpath(sanitized_name)
         os.makedirs(output_directory, exist_ok=True)
 
-        handler = logging.FileHandler(output_directory.joinpath(f'step.log'),
+        handler = logging.FileHandler(output_directory.joinpath('step.log'),
                                       mode='w')
         handler.setLevel(logging.DEBUG)
 
diff --git a/pw_watch/py/pw_watch/watch.py b/pw_watch/py/pw_watch/watch.py
index 5f3e067..1ff21db 100755
--- a/pw_watch/py/pw_watch/watch.py
+++ b/pw_watch/py/pw_watch/watch.py
@@ -221,9 +221,9 @@
 
         self.builds_succeeded = []
         num_builds = len(self.build_commands)
-        _LOG.info(f'Starting build with {num_builds} directories')
+        _LOG.info('Starting build with %d directories', num_builds)
         for i, cmd in enumerate(self.build_commands, 1):
-            _LOG.info(f'[{i}/{num_builds}] Starting build: {cmd}')
+            _LOG.info('[%d/%d] Starting build: %s', i, num_builds, cmd)
 
             # Run the build. Put a blank before/after for visual separation.
             print()
@@ -240,7 +240,8 @@
             else:
                 level = logging.ERROR
                 tag = '(FAIL)'
-            _LOG.log(level, f'[{i}/{num_builds}] Finished build: {cmd} {tag}')
+            _LOG.log(level, '[%d/%d] Finished build: %s %s', i, num_builds,
+                     cmd, tag)
             self.builds_succeeded.append(build_ok)
 
     # Implementation of DebouncedFunction.cancel()
@@ -384,7 +385,7 @@
             _die("Build directory doesn't exist: %s", build_target)
         else:
             _LOG.info(
-                f'Will build [{i}/{len(build_commands)}]: {build_target}')
+                'Will build [%d/%d]: %s', i, len(build_commands), build_target)
 
     _LOG.debug('Patterns: %s', patterns)