language/python: don't die in reads_brewed_pth_files?

Addresses #33900. Anything that causes an exception here is probably a symptom of a larger problem but it's probably bad for the doctor to die. :syringe:

The reported error was:

    Error: undefined method close! for nil:NilClass
    /usr/local/Library/Homebrew/extend/pathname.rb:129:in ensure in atomic_write
    ...

Is there a better way to catch this?
Assigning `result = nil` isn't strictly necessary, uninitialized locals are `nil` by default (and `begin` does not introduce a new scope).

If we go this route, we'll probably want to structure it more like:
 
```rb
probe_file = # ...
begin
  # ...
rescue
  # ...
ensure
  probe_file.unlink if probe_file.exist?
end
```

in that case the `result` local is unnecessary, as the last expression in the `begin` clause will be the return value of the method.