move config migration to its own file

tests/test_migrator.py

@@ -0,0 +1,57 @@
+"""
+Unit test  functions in installer.py
+"""
+import os
+from datetime import date
+
+from tljh import migrator, config
+
+
+def test_migrate_config(tljh_dir):
+    CONFIG_FILE = config.CONFIG_FILE
+    CONFIG_DIR = config.CONFIG_DIR
+    OLD_CONFIG_FILE = os.path.join(tljh_dir, "config.yaml")
+    OLD_CONFIG_D = os.path.join(tljh_dir, "jupyterhub_config.d")
+    CONFIG_D = os.path.join(config.CONFIG_DIR, "jupyterhub_config.d")
+    old_config_py = os.path.join(OLD_CONFIG_D, "upgrade.py")
+    new_config_py = os.path.join(CONFIG_D, "upgrade.py")
+
+    # initial condition: nothing exists
+    assert not os.path.exists(CONFIG_FILE)
+    assert not os.path.exists(OLD_CONFIG_FILE)
+    assert os.path.isdir(CONFIG_DIR)
+
+    # run migration with old config and no new config
+    upgraded_config = "old: config\n"
+    with open(OLD_CONFIG_FILE, "w") as f:
+        f.write(upgraded_config)
+    os.makedirs(OLD_CONFIG_D, exist_ok=True)
+    with open(old_config_py, "w") as f:
+        f.write("c.JupyterHub.log_level = 10")
+
+    migrator.migrate_config_files()
+    assert os.path.exists(CONFIG_FILE)
+    assert not os.path.exists(OLD_CONFIG_FILE)
+    with open(CONFIG_FILE) as f:
+        assert f.read() == upgraded_config
+    assert os.path.exists(new_config_py)
+    assert not os.path.exists(OLD_CONFIG_D)
+
+    # run again, this time with both old and new config
+    duplicate_config = "dupe: config\n"
+    with open(OLD_CONFIG_FILE, "w") as f:
+        f.write(duplicate_config)
+    migrator.migrate_config_files()
+    assert os.path.exists(CONFIG_FILE)
+    assert not os.path.exists(OLD_CONFIG_FILE)
+    # didn't clobber config:
+    with open(CONFIG_FILE) as f:
+        assert f.read() == upgraded_config
+
+    # preserved old config
+    backup_config = CONFIG_FILE + f".old.{date.today().isoformat()}"
+    assert os.path.exists(backup_config)
+    with open(backup_config) as f:
+        assert f.read() == duplicate_config
+
+    # migrate jupyterhub_con