fix f-string syntax error in code generation

[sisp]

I've fixed a bug that caused an f-string syntax error in the template compilation when a template, that imports a macro, contains curly braces in its name. The bug was caused by the code-generation of an f-string in which the template name is inserted, and when the template name contains curly braces and the generated code gets executed, Python treats the curly braces as f-string braces. I've changed the code-generation to construct the string differently, such that the affected substring is not an f-string. Instead, the different substrings are concatenated using the + operator.

• fixes f-string sytax error when importing macro in a template which filename is also a template #1792

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[sisp]

I don't think any of the errors were introduced by this PR.

[sisp]

I've rebased this PR onto the latest state of the 3.1.x branch. All tests are passing except pre-commit.ci - pr, but the reported errors originate from 3.1.x and weren't introduced by this PR.

@davidism (or any other maintainer) – may I ask you for a review? 🙏

[sisp]

I've performed another rebase onto the latest state of the 3.1.x branch. All tests are passing now.

[davidism]

I've refactored the fix so that the curly braces are escaped rather than trying to keep track of nested quote levels and string concatenation. I also fixed the test, as it didn't seem to trigger an error regardless of the fix. Also added comments explaining the code.