Closes issue #2790.

test.test_support.catch_warning() where showwarning() was not being set
properly.

triggered warnings should be captured. This allows the context manager to be
used to just prevent the internal state of the 'warnings' framework and thus
allow triggered warnings to be displayed.

version on OSX 10.4.10

Make the Python implementation of warnings compatible with the C implementation regarding non-callable showwarning

Use float constructors instead of magic code for float constants

the stack was being unwound by two levels instead of one each time.

An assertion in readline() would fail as data was already in the
internal buffer even though the socket was in unbuffered read mode.
That case is now handled.  More importantly, read() has been fixed to
not over-recv() and leave newly recv()d data in the _fileobject buffer.

The max() vs min() issue in read() is now gone.  Neither was correct.
On bounded reads, always ask recv() for the exact amount of data we
still need.

Candidate for backporting to release25-maint along with r62627.