Add Python version check and faulty test to confirm it's working (should fail on 3.6 build only)

features/core.feature

@@ -135,3 +135,7 @@ Feature: Basic reading and writing to a journal
         When we run "jrnl --diagnostic"
         Then the output should contain "jrnl"
         And the output should contain "Python"
+
+    Scenario: Version warning appears for versions below 3.7
+        When we run "jrnl --diagnostic"
+        Then the Python version warning should appear if our version is below "3.7"

features/steps/core.py

@@ -371,6 +371,17 @@ def list_journal_directory(context, journal="default"):
         for file in f:
             print(os.path.join(root, file))
 
+@then("the Python version warning should appear if our version is below {version}")
+def check_python_warning_if_version_low_enough(context, version):
+    import packaging.version
+    import platform
+
+    if packaging.version.parse(platform.python_version()) < packaging.version.parse(version):
+        out = context.stderr_capture.getvalue()
+        assert "THIS SHOULD FAIL ON 3.6 REMOVE WHEN CONFIRMING - WARNING: Python versions" in out
+    else:
+        assert True
+
 
 @then("fail")
 def debug_fail(context):

jrnl/cli.py

@@ -21,6 +21,7 @@
 
 import argparse
 import logging
+import packaging.version
 import platform
 import re
 import sys
@@ -313,6 +314,9 @@ def configure_logger(debug=False):
 
 
 def run(manual_args=None):
+    if packaging.version.parse(platform.python_version()) < packaging.version.parse('3.7'):
+        print("WARNING: Python versions below 3.7 are not supported by jrnl\n", file=sys.stderr)
+
     if manual_args is None:
         manual_args = sys.argv[1:]