Drop support for Python 3.6 (#1200)

Co-authored-by: Bouwe Andela <b.andela@esciencecenter.nl>

Author: valeriupredoi

doc/recipe/preprocessor.rst

@@ -31,7 +31,7 @@ Overview
 ========
 
 ..
-   ESMValTool is a modular ``Python 3.6+`` software package possessing capabilities
+   ESMValTool is a modular ``Python 3.7+`` software package possessing capabilities
    of executing a large number of diagnostic routines that can be written in a
    number of programming languages (Python, NCL, R, Julia). The modular nature
    benefits the users and developers in different key areas: a new feature

environment.yml

@@ -11,6 +11,6 @@ dependencies:
   - esmpy
   - iris>=3.0.2,<3.0.4  # iris=3.0.4 dropped support for Python 3.6 but we haven't just yet (23 July 2021, bugfix release 2.3.1)
   - graphviz
-  - python>=3.6
+  - python>=3.7
   - python-stratify
   - scipy<1.6  # until ESMValGroup/ESMValCore/issues/927 gets resolved

package/meta.yaml

@@ -29,13 +29,13 @@ build:
 requirements:
   build:
     - git
-    - python>=3.6
+    - python>=3.7
     # Normally installed via pip:
     - pytest-runner
     - setuptools_scm
   run:
     # esmvaltool
-    - python>=3.6
+    - python>=3.7
     - graphviz
     - iris>=3.0.2,<3.0.4  # iris=3.0.4 dropped support for Python 3.6 but we haven't just yet (23 July 2021, bugfix release 2.3.1)
     - python-stratify

setup.py

@@ -198,9 +198,9 @@ def read_description(filename):
         'Natural Language :: English',
         'Operating System :: POSIX :: Linux',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
         'Topic :: Scientific/Engineering',
         'Topic :: Scientific/Engineering :: Atmospheric Science',
         'Topic :: Scientific/Engineering :: GIS',

tests/integration/preprocessor/_io/test_concatenate.py

@@ -384,7 +384,7 @@ def test_fail_on_calendar_concatenate_with_overlap(self):
             Cube([33., 55.],
                  var_name='sample',
                  dim_coords_and_dims=((time_coord, 0), )))
-        with self.assertRaises(TypeError):
+        with self.assertRaises((TypeError, ValueError)):
             _io.concatenate(self.raw_cubes)
 
     def test_fail_on_units_concatenate_with_overlap(self):