`ggtitle` should accept keyword arguments

[krassowski]

Calling:

ggtitle(title='something')

Raises:

TypeError: ggtitle.__init__() got some positional-only arguments passed as keyword arguments: 'title'

This was introduced recently by 0b147f0

plotnine/plotnine/labels.py

Lines 139 to 150 in a3d893f

	class ggtitle(labs):
	"""
	Create plot title
	Parameters
	----------
	title :
	Plot title
	"""
	def __init__(self, title: str, /):
	self.title = title

ylab/xlab also have a similar issue but it is more notable for ggtitle because coming from ggplot I am used to calling ggtitle(title='something', subtitle='something else')