The ipaddress module in Python is an excellent tool, but I noticed it is missing a feature that I needed several months ago, which is the ability to find the next closest subnet with a specific prefix length.

For example, imagine I had a IPv4Network("10.10.0.72/30"), how would I find the next possible /25 address? It is not the most straightforward thing to do, so think it would be a great enhancement to the ipaddress library.

I think this can be achieved by adding in a new method to the BaseNetwork class, the method could be defined like "next_prefix(next_prefix=None)".  Calling this method would return an IPv4/v6 address that is the closest possible match with the new prefix (defined as next_prefix).

Example calls:
v4 = IPv4Network("10.10.0.72/30")
next_network = v4.next_subnet(next_prefix=25)
# Output: next_network = IPv4Network("10.10.0.128/25")

v4 = IPv4Network("10.10.0.72/30")
next_network = v4.next_subnet(next_prefix=30)
# Output: next_network = IPv4Network("10.10.0.76/30")

v4 = IPv4Network("10.10.0.72/30")
next_network = v4.next_subnet() # if next_prefix is not defined it will use the existing prefix of /30, so this call is exactly the same as the previous
# Output: next_network = IPv4Network("10.10.0.76/30")

v6 = IPv6Network("2001:db8:aaaa:aaaa:aaaa:aaaa:aaaa:0000/112")
next_network = v6.next_subnet()
# Output: next_network = IPv6Network("2001:db8:aaaa:aaaa:aaaa:aaaa:aaab:0/112")

v6 = IPv6Network("2001:db8:aaaa:aaaa:aaaa:aaaa:aaaa:0000/112")
next_network = v6.next_subnet(next_prefix=64)
# Output: next_network = IPv6Network("2001:db8:aaaa:aaab::/64")

I am going to be working on this and plan to raise a PR soon.  This is my first time contributing to Python, so I appreciate your help / comments / suggestions / guidance as I go along.