Compatibility issue with Python 3.7

[AiOO]

(This is not the same issue with as #116)

Since the typing module in Python 3.7 doesn't have _Union class, AttributeError occurs by below lines. To support Python 3.7 it should be fixed.

nirum-python/nirum/_compat.py

Lines 28 to 43 in 2f85525

	if hasattr(typing, '_Union'):
	def is_union_type(type_):
	return isinstance(type_, typing._Union)
	def get_union_types(type_):
	if is_union_type(type_):
	return type_.__args__
	else:
	def is_union_type(type_):
	return isinstance(type_, typing.UnionMeta)
	def get_union_types(type_):
	if is_union_type(type_):
	return type_.__union_params__ \
	if hasattr(type_, '__union_params__') \
	else type_.__args__

p.s. @kanghyojun Could you explain why you used _Union instead of Union?

Relative links

• Kill __subclasscheck__ python/typing#283: Removed UnionMeta and added _Union and Union
• bpo-32226: PEP 560: improve typing module python/cpython#4906: Removed _Union. Union is now using the new class _SpecialForm.

[kanghyojun]

It seems need to be fixed. But from 0.5.0, nirum-python nearly used for Python target. So why don't you re-create target package on new nirum compiler(0.5.0+).