Allow more Python scalar conversions

Allow more Python scalar conversions

PR python/18352 points out that the gdb Python code can't convert an
integer-valued gdb.Value to a Python float.  While writing the test I
noticed that, similarly, converting integer gdb.Values to float does
not work.  However, all of these cases seem reasonable.

gdb/ChangeLog
2018-09-23  Tom Tromey  <tom@tromey.com>

	PR python/18352;
	* python/py-value.c (valpy_float): Allow conversions from int or
	char.
	(valpy_int, valpy_long): Allow conversions from float.

gdb/testsuite/ChangeLog
2018-09-23  Tom Tromey  <tom@tromey.com>

	PR python/18352;
	* gdb.python/py-value.exp (test_float_conversion): New proc.
	Use it.