flake8: Add configuration file

This allows running flake8 on the code to check the formatting and some
common problems. The code is not currently compliant with flake8, so
flake8 is configured to ignore a lot of problems, but these could be
(gradually or selectively) fixed in the future.

This also tells flake8 to ignore the waf files, and tell it that `_` is
an additional builtin function (which is inserted into the builtin scope
by `hamster.lib.i18n.setup_i18n()` and `gettext.install()`).

Author: matthijskooijman

.flake8

@@ -0,0 +1,49 @@
+[flake8]
+exclude=wscript,waf,waflib
+builtins=_
+
+# This just ignores everything that currently fails. Over time, most of these
+# should probably be fixed in the code and removed here.
+extend-ignore =
+    E117, # over-indented
+    E122, # continuation line missing indentation or outdented
+    E124, # closing bracket does not match visual indentation
+    E125, # continuation line with same indent as next logical line
+    E127, # continuation line over-indented for visual indent
+    E128, # continuation line under-indented for visual indent
+    E129, # visually indented line with same indent as next logical line
+    E131, # continuation line unaligned for hanging indent
+    E202, # whitespace before ')'
+    E203, # whitespace before ':'
+    E211, # whitespace before '('
+    E221, # multiple spaces before operator
+    E222, # multiple spaces after operator
+    E225, # missing whitespace around operator
+    E227, # missing whitespace around bitwise or shift operator
+    E231, # missing whitespace after ':'
+    E251, # unexpected spaces around keyword / parameter equals
+    E261, # at least two spaces before inline comment
+    E262, # inline comment should start with '# '
+    E265, # block comment should start with '# '
+    E271, # multiple spaces after keyword
+    E272, # multiple spaces before keyword
+    E301, # expected 1 blank line, found 0
+    E302, # expected 2 blank lines, found 1
+    E303, # too many blank lines (2)
+    E305, # expected 2 blank lines after class or function definition, found 1
+    E306, # expected 1 blank line before a nested definition, found 0
+    E401, # multiple imports on one line
+    E402, # module level import not at top of file
+    E501, # line too long
+    E502, # the backslash is redundant between brackets
+    E701, # multiple statements on one line (colon)
+    E703, # statement ends with a semicolon
+    E711, # comparison to None should be 'if cond is not None:'
+    E712, # comparison to False should be 'if cond is False:' or 'if not cond:'
+    E722, # do not use bare 'except'
+    E731, # do not assign a lambda expression, use a def
+    F401, # 'xxx' imported but unused
+    F811, # redefinition of unused 'xxx' from line 32
+    F841, # local variable 'xxx' is assigned to but never used
+    W605, # invalid escape sequence
+