Fix what I think is a client bug

[erikbern]

This looks like a bug to me – closing any client resets the singleton.

Although the only impact is if you close clients then we'll recreate new clients unnecessarily. So fairly low impact.

@luiscape any memory of this? It's from PR #1030

[luiscape]

@erikbern You may be right. I looked but can't find the details. @thundergolfer would you remember if this is needed?

Thank you for fixing -- sorry for the bug!

[erikbern]

Hm I actually wonder if what we want to do is to remove the env client if that's the client that's being closed. I.e. something like this

if self is _Client._client_from_env:
    self.set_env_client(None)

Let me make that change!

[thundergolfer]

Yep that makes sense @erikbern. Alternatively the memory_snapshot() function can just call set_env_client directly? I think it's the only consumer of this behavior.