[refactor:job_backend] Use Slurm as the default job back-end

- The GC3Pie back-end for job management that has been the default so far
  is no longer actively supported. Use the Slurm back-end as default
  instead.
- Add deprecation message when the GC3Pie class is instantiated.

Issue: #4565

easybuild/tools/config.py

@@ -95,7 +95,7 @@
 DEFAULT_ENV_FOR_SHEBANG = '/usr/bin/env'
 DEFAULT_ENVVAR_USERS_MODULES = 'HOME'
 DEFAULT_INDEX_MAX_AGE = 7 * 24 * 60 * 60  # 1 week (in seconds)
-DEFAULT_JOB_BACKEND = 'GC3Pie'
+DEFAULT_JOB_BACKEND = 'Slurm'
 DEFAULT_JOB_EB_CMD = 'eb'
 DEFAULT_LOGFILE_FORMAT = ("easybuild", "easybuild-%(name)s-%(version)s-%(date)s.%(time)s.log")
 DEFAULT_MAX_FAIL_RATIO_PERMS = 0.5

easybuild/tools/job/gc3pie.py

@@ -104,6 +104,10 @@ def __init__(self, *args, **kwargs):
     def _check_version(self):
         """Check whether GC3Pie version complies with required version."""
 
+        deprication_msg = "The GC3Pie job back-end is no longer maintained and will be deprecated"
+        deprication_msg += ", please use a different backend"
+        _log.deprecated(deprication_msg, '5.0')
+
         try:
             from pkg_resources import get_distribution, DistributionNotFound
             pkg = get_distribution('gc3pie')