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

.flake8

@@ -11,12 +11,12 @@
 # - E272: multiple spaces before keyword
 #
 # Let people use terse Python features:
-# - E731 : lambda expressions
+# - E731: lambda expressions
 #
 # Spack allows wildcard imports:
 # - F403: disable wildcard import
 #
-# These are required to get the package.py files to test clean.
+# These are required to get the package.py files to test clean:
 # - F405: `name` may be undefined, or undefined from star imports: `module`
 # - F821: undefined name `name` (needed for cmake, configure, etc.)
 # - F999: syntax error in doctest