Attribute Tab Completion for Pandas Series

[tmastny]

This PR introduces potentially breaking changes to siuba, so it's further off than #248. However, I think this is really exciting and wanted to share this!

The main difference between this PR and #248 is that the output of Symoblic.__getattr__ is subclassed so it returns pandas.Series suggestions. Therefore, _ will provide completions based on the most recent dataframe and _.attribute will provide Series completions.

And since SymbolicAttr inherits (and is basically identical to) Symbolic, everything under the hood should work, but that's the part I need to do more digging into.

siuba/siuba/siu.py

Lines 664 to 671 in 0d6584c

	def __getattr__(self, x):
	# temporary hack working around ipython pretty.py printing
	#if x == "__class__": return Symbolic
	return SymbolicAttr(BinaryOp(
	"__getattr__",
	self.__source,
	strip_symbolic(x)

siuba/siuba/siu.py

Lines 731 to 733 in 0d6584c

	class SymbolicAttr(Symbolic):
	def __dir__(self):
	return dir(Series)

[tmastny]

Based on the progress in #248 I don't think this is the right approach anymore. #248 also supports Series completion, which is the most important thing.