Remove a lot of test references to Python 2/3.5

conftest.py

@@ -1,7 +1,6 @@
 import tempfile
 import shutil
 import os
-import sys
 from functools import partial
 
 import pytest
@@ -18,9 +17,6 @@ collect_ignore = [
     'build/',
     'test/examples',
 ]
-if sys.version_info < (3, 6):
-    # Python 2 not supported syntax
-    collect_ignore.append('test/test_inference/test_mixed.py')
 
 
 # The following hooks (pytest_configure, pytest_unconfigure) are used
@@ -45,7 +41,7 @@ def pytest_addoption(parser):
                      help="Warnings are treated as errors.")
 
     parser.addoption("--env", action='store',
-                     help="Execute the tests in that environment (e.g. 35 for python3.5).")
+                     help="Execute the tests in that environment (e.g. 39 for python3.9).")
     parser.addoption("--interpreter-env", "-I", action='store_true',
                      help="Don't use subprocesses to guarantee having safe "
                           "code execution. Useful for debugging.")
@@ -180,19 +176,3 @@ def skip_pre_python37(environment):
         # This if is just needed to avoid that tests ever skip way more than
         # they should for all Python versions.
         pytest.skip()
-
-
-@pytest.fixture()
-def skip_pre_python35(environment):
-    if environment.version_info < (3, 5):
-        # This if is just needed to avoid that tests ever skip way more than
-        # they should for all Python versions.
-        pytest.skip()
-
-
-@pytest.fixture()
-def skip_pre_python36(environment):
-    if environment.version_info < (3, 6):
-        # This if is just needed to avoid that tests ever skip way more than
-        # they should for all Python versions.
-        pytest.skip()