- Don't require numpy 2.

- update to 6.1.2:
* When reading CDS and MRT files, only interpret a line as a
section delimiter if it contains exclusively dashes or equal
signs. This enables rows starting with dashes.
* Fix a spurious exception when reading integer compressed
images with blanks.
* Fixed a crash that occurred for files opened via
fits.open(..., mode='update'), on Windows, and with numpy 2.0
installed. A warning is now emitted in cases most likely to
escalate into undefined behavior (e.g., segfaults), i.e.,
when a closed memory map object is still referenced by
external code. Please report any regression found.
* Fixed a bug that caused models returned by non-linear fitters
to have sync_constraints set to False, which caused
constraints accessed through, e.g., Model.fixed to not be in
sync with the fixed attribute of the parameters.
* Fixed a bug that caused CompoundModel.without_units_for_data
to return an incorrectly constructed model when the compound
model contained a * or / operation, and which also caused
fitting to not work correctly with compound models that
contained * or / operations.
* The OGIP parser is now less restrictive with strings that
represent a unit that includes the sqrt function. For
example, u.Unit("sqrt(m)**3", format="ogip") no longer causes
a ValueError.
* Fixed an edge-case bug in overlap_slices where the function
could return an empty slice for non-overlapping slices.
* Fixed a WCSAxes bug when overlaying a frame with default