_BaseNetwork contained (faulty) methods for creating string representations.
I've fixed them and put them to use by eliminating identical overrides.

ipaddress._BaseV4.is_unspecified() compared IP addresses using "in" which
fails.

break someone's over specified test that depends on the exact error message.

when the path existed and had the S_ISGID mode bit set when it was
not explicitly asked for.  This is no longer an exception as mkdir
cannot control if the OS sets that bit for it or not.

when the path existed and had the S_ISGID mode bit set when it was not
explicitly asked for.  This is no longer an exception as mkdir cannot control
if the OS sets that bit for it or not.

Issue 14673: Add sys.implementation

'name' to 'expression' as the latter is what the string actually represents.
The bug in this issue was only catching NameError and AttributeError when
evaluating an expression that was not necessarily a name.

Code patch by Roger Serwy.

This is a behavior change: before this leading and trailing spaces were
stripped from ASCII parts, now they are preserved.  Without this fix we didn't
parse the examples in the RFC correctly, so I think breaking backward
compatibility here is justified.

Patch by Ralf Schlatterbeck.