Try to use the same python for running diagnostics as for esmvaltool

[bouweandela]

Before you start, please read CONTRIBUTING.md.

Tasks

• Create an issue to discuss what you are going to do, if you haven't done so already (and add the link at the bottom)
• This pull request has a descriptive title that can be used in a changelog
• Add unit tests
• Circle/CI tests pass. Status can be seen below your pull request. If the tests are failing, click the link to find out why.
• Codacy code quality checks pass. Status can be seen below your pull request. If there is an error, click the link to find out why. If you suspect Codacy may be wrong, please ask by commenting.

If you need help with any of the tasks above, please do not hesitate to ask by commenting in the issue or pull request.

---

Closes #653

[valeriupredoi]

slightly suspicious about this, see comment

[mattiarighi]

I tried this with recipe_python.yml but I get an error:

2020-06-05 10:06:38,015 UTC [41426] ERROR   Failed to run concatenate([<iris 'Cube' of precipitation_flux / (kg m-2 s-1) (time: 1872; latitude: 64; longitude: 128)>], {})
2020-06-05 10:06:39,040 UTC [41384] INFO    Maximum memory used (estimate): 0.7 GB
2020-06-05 10:06:39,041 UTC [41384] INFO    Sampled every second. It may be inaccurate if short but high spikes in memory consumption occur.
2020-06-05 10:06:39,043 UTC [41384] ERROR   Program terminated abnormally, see stack trace below for more information
multiprocessing.pool.RemoteTraceback:
"""
Traceback (most recent call last):
  File "/pf/b/b309057/SOFTWARE/miniconda3/envs/esmvaltool/lib/python3.8/multiprocessing/pool.py", line 125, in worker
    result = (True, func(*args, **kwds))
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/_task.py", line 723, in _run_task
    output_files = task.run()
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/_task.py", line 242, in run
    self.output_files = self._run(input_files)
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/preprocessor/__init__.py", line 428, in _run
    product.apply(step, self.debug)
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/preprocessor/__init__.py", line 296, in apply
    self.cubes = preprocess(self.cubes, step, **self.settings[step])
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/preprocessor/__init__.py", line 238, in preprocess
    result.append(_run_preproc_function(function, items, settings))
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/preprocessor/__init__.py", line 224, in _run_preproc_function
    return function(items, **kwargs)
  File "/mnt/lustre01/pf/b/b309057/ESMValTool/core/esmvalcore/preprocessor/_io.py", line 193, in concatenate
    _fix_aux_factories(result)
UnboundLocalError: local variable 'result' referenced before assignment
"""

[valeriupredoi]

@mattiarighi need to merge master, this issue was resolved after this branch was created 🍺