update md5 to use hashlib or other standard module

[mkolopanis]

as per PEP memo https://www.python.org/dev/peps/pep-0004/ the md5 module is depreciated and should be replaced with hashlib. This is also required for conversion to python 3 in issue #131

[mkolopanis]

ah I'm also noticing now that the function which uses the module claims itself to be depreciated

hera_pspec/hera_pspec/utils.py

Lines 15 to 20 in d2de2ff

	def hash(w):
	"""
	Return an MD5 hash of a set of weights.
	"""
	DeprecationWarning("utils.hash is deprecated.")
	return md5.md5(w.copy(order='C')).digest()

[philbull]

I have removed this function in PR #189.