Fix the sherpa installation

[bkhelifi]

Gammapy version
gammapy 1.2, under Linux

Bug description
The installation of the environment with mamba/conda fails because of sherpa. This has been reported by several users.

      checking dependency style of gcc... gcc3
      checking for bison... no
      checking for byacc... no
      checking for yacc... no
      configure: error: bison/byacc/yacc not found.
                Please install bison
      configure: error: ./configure failed for region-4.14
      [end of output]
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
  ERROR: Failed building wheel for sherpa

Even afterwards after an installation without sherpa, the command pip install sherpa==4.16 fails.

[registerrier]

One option here is to remove the version constraint on sherpa, or use 4.15 instead (in which case conda should work).

[Astro-Kirsty]

Do we know what versions of Linux this happens for?
Just curious, because I did not encounter this with Ubuntu 22.04

[bkhelifi]

I have the same ubuntu version.
Within the env of the version 1.2, pip install sherpa==4.15 leads to the same error...
I have in the output "configure: error: bison/byacc/yacc not found" for the configuration of region-4.14

[vmharvey]

I have the same issue on Ubuntu 22.04.4. I found a bug on the sherpa issue tracker which shows the same problem occurring: sherpa/sherpa#828. From this comment sherpa/sherpa#828 (comment), assuming it's still true after a few years, it appears that installing sherpa via pip triggers a source install, which requires the user to have all the sherpa dependencies, including the build system bison.

Modifying the env file by removing sherpa==4.16 from the pip section and putting sherpa=4.15 in the conda section fixed the problem for me:

  - tqdm
  - sherpa=4.15
  - pip:
    - ray[default]==2.9

It appears that the official sherpa channel on anaconda only has version 4.15, hence the small downgrade required.

[Astro-Kirsty]

Sherpa was initially moved to the pip installation due to both apple and windows systems not being able to install it through this method.
#4983 (comment)
As a user of these systems sherpa cannot be installed outside pip, so this line of the environment file needs to be removed.

Not sure how we wish to proceed here?
Appears to be some issue with any combination with at least one operating system :(