Properly ignore flake8 F811 redefinition errors (#3932)

* Properly ignore flake8 F811 redefinition errors
* Add unit tests for flake8 command
* Allow spack flake8 to work on systems with older git
* Skip flake8 unit tests for Python 2.6 and 3.3

var/spack/repos/builtin/packages/metis/package.py

@@ -84,7 +84,7 @@ def patch(self):
             filter_file('#define MAX_JBUFS 128', '#define MAX_JBUFS 24',
                         join_path(source_path, 'GKlib', 'error.c'))
 
-    @when('@:4')  # noqa: F811
+    @when('@:4')
     def install(self, spec, prefix):
         # Process library spec and options
         if any('+{0}'.format(v) in spec for v in ['gdb', 'int64', 'real64']):
@@ -175,7 +175,7 @@ def install(self, spec, prefix):
             Executable(test_bin('mesh2dual'))(test_graph('metis.mesh'))
             """
 
-    @when('@5:')  # noqa: F811
+    @when('@5:')
     def install(self, spec, prefix):
         source_directory = self.stage.source_path
         build_directory = join_path(source_directory, 'build')

var/spack/repos/builtin/packages/qt/package.py

@@ -251,7 +251,7 @@ def common_config_args(self):
     # Don't disable all the database drivers, but should
     # really get them into spack at some point.
 
-    @when('@3')  # noqa: F811
+    @when('@3')
     def configure(self):
         # A user reported that this was necessary to link Qt3 on ubuntu.
         # However, if LD_LIBRARY_PATH is not set the qt build fails, check
@@ -268,7 +268,7 @@ def configure(self):
                   '-release',
                   '-fast')
 
-    @when('@4')  # noqa: F811
+    @when('@4')
     def configure(self):
         configure('-fast',
                   '-{0}gtkstyle'.format('' if '+gtk' in self.spec else 'no-'),
@@ -276,7 +276,7 @@ def configure(self):
                   '-arch', str(self.spec.architecture.target),
                   *self.common_config_args)
 
-    @when('@5.0:5.6')  # noqa: F811
+    @when('@5.0:5.6')
     def configure(self):
         webkit_args = [] if '+webkit' in self.spec else ['-skip', 'qtwebkit']
         configure('-no-eglfs',
@@ -284,7 +284,7 @@ def configure(self):
                   '-{0}gtkstyle'.format('' if '+gtk' in self.spec else 'no-'),
                   *(webkit_args + self.common_config_args))
 
-    @when('@5.7:')  # noqa: F811
+    @when('@5.7:')
     def configure(self):
         config_args = self.common_config_args