fix: python template for bundling

[smichaud]

Fixes #

When bundling using Nuitka or PyInstaller, the template.txt is not bundled which leads to a runtime error FileNotFoundError.

Changes in this PR

• Use a Python file with a constant instead of template.txt
• Removed the template cached property since it is now a constant

cc/ @klen

[klen]

@smichaud Thank you!

Why do you comment the template content? It broke migrations.

[smichaud]

@klen You are right, I commented lines while exploring if it solved my problem. I see that you fixed it before merging. Thank you!