Modernize the code a little

[timj]

• Supports python 3.13 and numpy 2
• Starts dropping python 2 and pyfits support.

The libyaml detection doesn't seem to work (and doesn't work on the master branch with python 3.11) and there is a test failure on master even before this work:

======================================================================
FAIL: test_ZoomMap (__main__.TestAst.test_ZoomMap)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/timj/work/starlink-pyast/starlink/ast/test/test.py", line 180, in test_ZoomMap
    self.assertEqual(zoommap.Nobject, 1, str(zoommap))
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 3 != 1 :  Begin ZoomMap 	# Zoom about the origin
    Nin = 1 	# Number of input coordinates
 IsA Mapping 	# Mapping between coordinate systems
    Zoom = 1.2 	# Zoom factor
 End ZoomMap

With python 3.13 there is a new warning:

starlink/ast/test/test.py::TestAst::test_MyChannel
  /Users/timj/work/tmp/py13/lib/python3.13/site-packages/_pytest/unraisableexception.py:85: PytestUnraisableExceptionWarning: Exception ignored in PyObject_HasAttrString(); consider using PyObject_HasAttrStringWithError(), PyObject_GetOptionalAttrString() or PyObject_GetAttrString(): None
  
  Traceback (most recent call last):
    File "/Users/timj/work/starlink-pyast/starlink/ast/test/test.py", line 950, in test_MyChannel
      channel = starlink.Ast.Channel(ss, ss)
  TypeError: The supplied 'source' object does not have an 'astsource' method and is not a sequence.

[timj]

I also ran black and isort on the code and did some automated updates with ruff. Also use context managers for open().

I'l fix the travis and move it to GitHub actions.