Fix flake8 failures

[thopre]

Summary

Fix flake8 failures when running tox and add a note in the README to run
tox for testing patches.

[tnfchris]

Thanks,

Can we not run tox in the CI?

[thopre]

In D112304#3080351, @tnfchris wrote:

Thanks,

Can we not run tox in the CI?

Good question, I'm not too sure where this gets configured. I'll ask around.

README.md

 
  $ROOT/lnt/server/db - Database schema, utilities, and examples of the LNT plist format.
 
  $ROOT/docs - Sphinx documentation for LNT.
 
  $ROOT/tests - Tests for the infrastructure.
 
 For more information, see the web documentation, or docs/.
+
+Testing
+=======
+
+Testing is done by running tox from the top-level directory. It runs the tests
+for both Python 2 and Python 3, it also checks code style.

lnt/formats/__init__.py

 """
 Utilities for converting to LNT's test format.
 
 LNT formats are described by dictionaries with 'name', 'read', and 'write'
 fields. Only the 'name' field is required. The 'read' field should be a
 callable taking a path_or_file object, the 'write' function should be a
 callable taking a Python object to write, and the path_or_file to write to.
 """
 
 from __future__ import absolute_import
 from typing import List, Dict
 try:
-    from plistlib import readPlist
+    from plistlib import readPlist  # noqa: F401  # used to test exception
     from .PlistFormat2 import format as plist  # for Python 2
 except ImportError:
     from .PlistFormat import format as plist  # for Python 3
 from .JSONFormat import format as json
 
 formats = [plist, json]  # type: List[Dict]
 formats_by_name = dict((f['name'], f) for f in formats)  # type: Dict[str, Dict]
 format_names = list(formats_by_name.keys())  # type: List[str]

lnt/testing/profile/profile.py

def fromFile(f):
         """
         Load a profile from a file.
         """
         for impl in lnt.testing.profile.IMPLEMENTATIONS.values():
             if impl.checkFile(f):
                 ret = None
                 with open(f, 'rb') as fd:
                     if impl is lnt.testing.profile.perf.LinuxPerfProfile:
-                        ret = impl.deserialize(fd,
-                                  nm = os.getenv('CMAKE_NM', 'nm'),
-                                  objdump = os.getenv('CMAKE_OBJDUMP', 'objdump'),
-                                  binaryCacheRoot = os.getenv('LNT_BINARY_CACHE_ROOT', ''))
+                        ret = impl.deserialize(
+                            fd,
+                            nm=os.getenv('CMAKE_NM', 'nm'),
+                            objdump=os.getenv('CMAKE_OBJDUMP', 'objdump'),
+                            binaryCacheRoot=os.getenv('LNT_BINARY_CACHE_ROOT', ''))
                     else:
                         ret = impl.deserialize(fd)
                 if ret:
                     return Profile(ret)
                 else:
                     return None
         raise RuntimeError('No profile implementations could read this file!')