Use Jinja sandboxed environment

[amanda11]

Use Jinja sandboxed environment, so that any templates that attempt to access insecure code will be rejected.

[m4dcoder]

• Do we need to update the Jinja version to >= 2.11 in requirements.txt?
• Can you also update description of this PR on why we are using sandboxed environment
• Please add a changelog entry.
• The CI weren't building because workflows was automatically disabled by Github due to inactivity. I believe there's a build failure that need to be fixed.ding because workflows was automatically disabled by Github due to inactivity. It's resolved now. But I believe there's an unrelated build failure that need to be fixed.

[amanda11]

• Do we need to update the Jinja version to >= 2.11 in requirements.txt?
• Can you also update description of this PR on why we are using sandboxed environment
• Please add a changelog entry.
• The CI weren't building because workflows was automatically disabled by Github due to inactivity. I believe there's a build failure that need to be fixed.ding because workflows was automatically disabled by Github due to inactivity. It's resolved now. But I believe there's an unrelated build failure that need to be fixed.

@m4dcoder Addressed your review comments (stealing the sphinx change from your PR!)

[m4dcoder]

Thank you!