fix!: use full import path to safe_lxml (#324)

Previously, edx-platform's safe_lxml module was imported as:

  import safe_lxml

This still works, but will stop working very soon.

This commit updates edx-sga's usage of safe_lxml to the new,
correct import path:

  import openedx.core.lib.safe_lxml

(Details & reasoning behind this import path change:
  * https://discuss.openedx.org/t/breaking-apart-edx-platforms-common-lib-folder/7556
  * https://openedx.atlassian.net/browse/BOM-2579)

This change is NOT backwards-compatible with Nutmeg or earlier.

edx_sga/sga.py

@@ -25,7 +25,7 @@
 from django.utils.timezone import now as django_now
 from django.utils.translation import gettext as _
 from lms.djangoapps.courseware.models import StudentModule
-from safe_lxml import etree
+from openedx.core.lib.safe_lxml import etree
 from common.djangoapps.student.models import user_by_anonymous_id
 from submissions import api as submissions_api
 from submissions.models import StudentItem as SubmissionsStudent

edx_sga/tests/test_sga.py

@@ -116,7 +116,7 @@ def fake_import(name, *args, **kwargs):
                 for module in ("common", "courseware", "lms", "xmodule"):
                     if name.startswith(f"{module}.") or name == module:
                         return mock.Mock()
-                if name == "safe_lxml":
+                if name == "openedx.core.lib.safe_lxml":
                     return real_import("lxml", *args, **kwargs)
                 raise