replace imghdr by puremagic

imghdr allowed to detect a few imagetimes and has been deprecated
in python 3.13: https://docs.python.org/3.12/library/imghdr.html

from the release notes:

> The filetype, puremagic, or python-magic libraries should be used as replacements.
> For example, the puremagic.what() function can be used to replace the imghdr.what()
> function for all file formats that were supported by imghdr.

See also https://github.com/cdgriffith/puremagic/blob/763349ec4d02ba930fb1142c6eb684afdf06c6ab/puremagic/main.py#L421

lib/galaxy/dependencies/pinned-requirements.txt

@@ -86,6 +86,7 @@ h5py==3.11.0 ; python_version >= "3.8" and python_version < "3.9"
 h5py==3.12.1 ; python_version >= "3.9" and python_version < "3.13"
 humanfriendly==10.0 ; python_version >= "3.8" and python_version < "3.13"
 idna==3.10 ; python_version >= "3.8" and python_version < "3.13"
+puremagic=1.28 ; python_version >= "3.8" and python_version < "3.13"
 importlib-metadata==8.5.0 ; python_version >= "3.8" and python_version < "3.13"
 importlib-resources==6.4.5 ; python_version >= "3.8" and python_version < "3.9"
 isa-rwval @ git+https://github.com/nsoranzo/isa-rwval.git@3d989181058d2765a93cb0e7ca85d6955e0eb6ef ; python_version >= "3.8" and python_version < "3.13"

lib/galaxy/util/image_util.py

@@ -25,8 +25,6 @@ def image_type(filename: str) -> Optional[str]:
             # We continue to try with imghdr, so this is a rare case of an
             # exception we expect to happen frequently, so we're not logging
             pass
-    if not fmt:
-        fmt = imghdr.what(filename)
     if fmt:
         return fmt.upper()
     else: