Use is/is not to check for None in python code.

Use is/is not to check for None in python code.

While reviewing a patch sent to the mailing list, I noticed there are few
places where python code checks if a variable is 'None' or not by using the
comparison operators '==' and '!='.  PEP8[1], which is used as coding standard
in GDB coding standards, recommends using 'is' / 'is not' when comparing to a
singleton such as 'None'.

This patch proposes to change the instances of '== None' by 'is None' and
'!= None' by 'is not None'.

[1] https://www.python.org/dev/peps/pep-0008/

gdb/doc/ChangeLog:

	* python.texi (Writing a Pretty-Printer): Use 'is None' instead of
	'== None'.

gdb/ChangeLog:

	* python/lib/gdb/FrameDecorator.py (FrameDecorator): Use 'is None' instead of
	'== None'.
	(FrameVars): Use 'is not None' instead of '!= None'.
	* python/lib/gdb/command/frame_filters.py (SetFrameFilterPriority): Use 'is None'
	instead of '== None' and 'is not None' instead of '!= None'.

gdb/testsuite/ChangeLog:

	* gdb.base/premature-dummy-frame-removal.py (TestUnwinder): Use
	'is None' instead of '== None' and 'is not None' instead of
	'!= None'.
	* gdb.python/py-frame-args.py (lookup_function): Same.
	* gdb.python/py-framefilter-invalidarg.py (Reverse_Function): Same.
	* gdb.python/py-framefilter.py (Reverse_Function): Same.
	* gdb.python/py-nested-maps.py (lookup_function): Same.
	* gdb.python/py-objfile-script-gdb.py (lookup_function): Same.
	* gdb.python/py-prettyprint.py (lookup_function): Same.
	* gdb.python/py-section-script.py (lookup_function): Same.
	* gdb.python/py-unwind-inline.py (dummy_unwinder): Same.
	* gdb.python/python.exp: Same.
	* gdb.rust/pp.py (lookup_function): Same.