#14984: On POSIX, enforce permissions when reading default .netrc.

Initial patch by Bruno Piguet.

This is implemented as if a useful .netrc file could exist without passwords,
which is possible in the general case; but in fact our netrc implementation
does not support it.  Fixing that issue will be an enhancement.
Merge #14984: On POSIX, enforce permissions when reading default .netrc.
Issue #18873: IDLE, 2to3, and the findnocoding.py script now detect Python
source code encoding only in comment lines.