Upgrading fermipy to numpy>2 and astropy>7

[facero]

This PR proposes to fix remaining issues with numpy2 and also issues that I discovered with astropy 6&7.

This follows the PR fixing numpy2 in the fermitools and this PR should be tested in a numpy and astropy 7 environment in conjunction with the fermitools numpy2 version.

Issues solved:

All issues (for now) were related to catalog.py.
I haven't yet tested a longer fermipy analysis than just gta.setup().
I will try later and report if any other bugs are found.

So far this is what I've done.

Numpy 2 related

As numpy2 deprecated the np.string_ call here was failing.

the fix was to use : (np.bytes_, np.str_) to handle both string and bytes columns
Maybe just np.str_ was enough. But at least we should be safe with both.

Astropy >6 related

In astropy >6 , table.join() here strictly rejects missing values in key columns. The join_tables() function in catalog.py was calling join() directly on tables where the left key column (Extended_Source_Name) had masked/NaN values for non-extended sources, causing:
astropy.table.np_utils.TableMergeError: Left key column 'Extended_Source_Name' has missing values

The fix here is more painful and I'm not completly sure that the proposed solution is the best.
The solution is to add a pre-processing step before the join() call that fills masked values in the key column for both tables with 0 or empty strings.

Maybe this entire join_tables function could be revisited and simplified as it is a bit cryptic now.
But let's try this fix first.

[facero]

if the tests are running with numpy 1.26, they might fail.

One would need to upgrade the CI environment to astropy >7, numpy 2>, gammapy>2

[facero]

      The following packages are incompatible
      ├─ fermitools >=2.2.0 * is installable and it requires
      │  └─ python >=3.11,<3.12.0a0 *, which can be installed;
      └─ python =3.9 * is not installable because it conflicts with any installable versions previously reported.

python 3.9 could be removed from CI

[ndilalla]

Thanks @facero! I just dropped support for python 3.9 on master. I see though that tests are failing due to:

ImportError: cannot import name 'pixel_scale_angle_at_skycoord' from 'regions._utils.wcs_helpers' (/Users/runner/miniconda3/envs/fermipy/lib/python3.11/site-packages/regions/_utils/wcs_helpers.py)

which is called by gammapy/utils/regions.py. Yesterday region 0.12 was released and gammapy looks not compatible yet. Here you can find the changelog: https://astropy-regions.readthedocs.io/en/stable/changelog.html#changelog