Incorporate import_expression logic into jishaku.

[Sachaa-Thanasius]

Rationale

import_expression is effectively unmaintained and isn't functional on python3.12 (according the issue linked below). The new logic should be functional on 3.8+.

Closes #228.

Summary of changes made

Adds a token stream transformer and AST transformer and incorporates them into a parse function with the same signature as ast.parse to achieve a similar end result to import_expression.parse. Also added tests, mostly sourced from import_expression and modified to account for the different and much smaller API surface.

Checklist

• [x] This PR changes the jishaku module/cog codebase
  • [x] These changes add new functionality to the module/cog
  • [x] These changes fix an issue or bug in the module/cog
  • [x] I have tested that these changes work on a production bot codebase
  • [x] I have tested these changes against the CI/CD test suite
  • [ ] I have updated the documentation to reflect these changes
• [ ] This PR changes the CI/CD test suite
  • [ ] I have tested my suite changes are well-formed (all tests can be discovered)
  • [ ] These changes adjust existing test cases
  • [ ] These changes add new test cases
• [ ] This PR changes prose (such as the documentation, README or other Markdown/RST documents)
  • [ ] I have proofread my changes for grammar and spelling issues
  • [ ] I have tested that any changes regarding Markdown/RST syntax result in a well formed document

[Sachaa-Thanasius]

Regarding the remaining points:

• I have a bot, but depending on the meaning of "production", it might not fill that criteria? It's a small one just for running jishaku, and it's currently using my branch on 3.12. The import syntax seems to be working fine so far.
  • EDIT: See the attached image below.
• For tests, I hope running python -m pytest for py3.8-3.12 locally will be enough on my end for now.
• For documentation, I am currently imagining a single bullet point in the changelog for 2.5.3 that says something along the lines of, "Implemented import_expressions functionality within Jishaku and fixed it to work on 3.8 - 3.12". Would this suffice?

@Gorialis

[Sachaa-Thanasius]

On a tangential note: it was mentioned in the related issue that the inline import feature should pass "the litmus test" on 3.8-3.12, but in the overall tracking issue for the next release (#212), it was also mentioned that jishaku would get a bump to 3.10. Is the latter still true, and if so, do I only have to care about this being compatible with 3.10+?

[Sachaa-Thanasius]

Since ioistired is back, I'll close this PR once upstream has been fixed.