#7152: Clarify that ProxyHandler is added only if proxy settings are detected.

Behavior confirmation and initial patch by Jessica McKellar.
Reflow paragraphs.

I know one of them looks worse, but now it is <80 chars and
matches the 2.7 text.
Issue #17358: imp.load_source() and load_compiled() should now return
modules which will typically work when reloaded.

A hack is used to support these functions as their API allowed them to
pass in a file object but then operate as if import had loaded them.
Unfortunately the hack kept a reference around for the file object
passed in which would be unusable on reload since it had been closed.
The solution is to simply use the hack for the initial load but then a
proper loader on the module so that imp.reload() at least has a chance
to work.
#7152: Remove incorrectly added reference to DataHandler.
raise an ImportError (rather than fatal) when __import__ is not found in __builtins__ (closes #17867)