fix: dashboard iframe to markdown db migration

[dpgaspar]

SUMMARY

Fixing a reported issue on a db migration with the following error:

INFO  [alembic.runtime.migration] Running upgrade f2672aa8350a -> 978245563a02, Migrate iframe in dashboard to markdown component
scanning dashboard (1/2) dashboard: 19 >>>>
ERROR [root] dashboard 19 has error: dictionary keys changed during iteration
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/superset/migrations/versions/978245563a02_migrate_iframe_to_dash_markdown.py", line 135, in upgrade
    for key, chart_position in position_dict.items():
RuntimeError: dictionary keys changed during iteration

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-io]

Codecov Report

Merging #11845 (8bcd59e) into master (4666445) will decrease coverage by 6.62%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #11845      +/-   ##
==========================================
- Coverage   67.12%   60.50%   -6.63%     
==========================================
  Files         915      872      -43     
  Lines       44537    42925    -1612     
  Branches     4235     3749     -486     
==========================================
- Hits        29897    25972    -3925     
- Misses      14526    16953    +2427     
+ Partials      114        0     -114     

Flag	Coverage Δ
cypress	55.08% <ø> (+3.47%)	⬆️
javascript	?
python	63.29% <0.00%> (-0.81%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ns/978245563a02_migrate_iframe_to_dash_markdown.py	0.00% <0.00%> (ø)
...uperset-frontend/src/dashboard/util/dnd-reorder.js	0.00% <0.00%> (-100.00%)	⬇️
...rset-frontend/src/dashboard/util/getEmptyLayout.js	0.00% <0.00%> (-100.00%)	⬇️
...dashboard/components/resizable/ResizableHandle.jsx	0.00% <0.00%> (-100.00%)	⬇️
.../src/dashboard/util/getFilterScopeFromNodesTree.js	0.00% <0.00%> (-93.48%)	⬇️
...src/dashboard/components/gridComponents/Header.jsx	10.52% <0.00%> (-86.85%)	⬇️
...rc/dashboard/components/gridComponents/Divider.jsx	13.33% <0.00%> (-86.67%)	⬇️
superset/db_engines/hive.py	0.00% <0.00%> (-85.72%)	⬇️
...ontend/src/dashboard/util/getDashboardFilterKey.ts	14.28% <0.00%> (-85.72%)	⬇️
...nd/src/views/CRUD/data/query/QueryPreviewModal.tsx	14.70% <0.00%> (-82.97%)	⬇️
... and 338 more

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[villebro]

LGTM, thanks for fixing! Minimal non-blocking nit.

[graceguo-supercat]

LGTM. Thanks for fixing it.

[graceguo-supercat]

Just curious...why I didn't hit this error while testing it (python 3.7)?

[villebro]

@graceguo-supercat AFAIK python 3.8 is more strict about this than previous versions.