Use Python 2.[67] / 3.X / PEP 3118 buffer protocol

This patch removes the non-IS_PY3K code in infpy_write_memory()
and infpy_search_memory().  In both cases, the remaining code
from these ifdefs is related to use of the PEP 3118 buffer protocol.
(Deleted code is either due to simplification or related to use of the
old buffer protocol.)  PEP 3118 is sometimes referred to as the "new"
buffer protocol, though it's not that new anymore.

The link below describes new features in Python 2.6.  In particular,
it says that the buffer protocol described by PEP 3118 is in Python
2.6.  It also says (at the top of the page) that Python 2.6 was
released on Oct 1, 2008.

https://docs.python.org/3/whatsnew/2.6.html#pep-3118-revised-buffer-protocol

The last security release for the Python 2.6 series was 2.6.9.  It was
released on Oct 29, 2013.  According to this document...

https://www.python.org/download/releases/2.6.9/

...support for the 2.6 series has ended:

    With the 2.6.9 release, and five years after its first release,
    the Python 2.6 series is now officially retired.  All official
    maintenance for Python 2.6, including security patches, has ended.
    For ongoing maintenance releases, please see the Python 2.7
    series.

As noted earlier, Python 2.6, Python 2.7, and Python 3.X all have
support for the PEP 3118 buffer protocol.  Python releases prior
to 2.6 use an older buffer protocol.  Since Python 2.6 has been
retired for a good while now, it seems reasonable to me to remove
code using the older buffer protocol from GDB.

I have also simplified some of the code via use of the Py_buffer
unique_ptr specialization which I introduced in the two argument
gdb.Value constructor patch series.  Therefore, there is a dependency
on patch #1 from that series.

I have tested against both Python 2.7.15 and 3.7.2.  I see no
regressions among the non-racy tests.  I've also verified that
PyBuffer_Release is being called when the affected functions exit
while running the tests in gdb.python/py-inferior.exp by hand.  I've
also tried running valgrind on GDB while running this test, but I'm
puzzled by the results that I'm seeing - I'm seeing no additional
leaks when I comment out the Py_buffer_up lines that I introduced.
That said, I'm not seeing any leaks that obviously originate from
either infpy_write_memory() or infpy_search_memory().

gdb/ChangeLog:

	* python/py-inferior.c (infpy_write_memory): Remove non-IS_PY3K
	code from these functions.  Remove corresponding ifdefs.  Use
	Py_buffer_up instead of explicit calls to PyBuffer_Release.
	Remove gotos and target of gotos.
	(infpy_search_memory): Likewise.

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