[TIDY FIRST] Remove conditional `typing_extensions` import for python < 3.8

[QMalcolm]

This repository doesn't currently, and never has, supported python versions less than 3.8. The conditional import being removed here is a holdover from when this code was cut out of dbt-core (which no longer supports python less than 3.8 either).

Checklist

• [X] I have read the contributing guide and understand what's expected of me
• [X] I have signed the CLA
• [X] I have run this code in development and it appears to resolve the stated issue
• [ ] ~I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR~
• [ ] ~I have run changie new to create a changelog entry~

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 56.37%. Comparing base (3b61b6f) to head (0ba69e6).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #134 +/- ## ======================================= Coverage 56.37% 56.37% ======================================= Files 50 50 Lines 3090 3088 -2 ======================================= - Hits 1742 1741 -1 + Misses 1348 1347 -1 ``` | [Flag](https://app.codecov.io/gh/dbt-labs/dbt-common/pull/134/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/dbt-labs/dbt-common/pull/134/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `56.37% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs#carryforward-flags-in-the-pull-request-comment) to find out more.

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[QMalcolm]

Do we still support python 3.8 dbt-core 1.8?

We do still support python 3.8 in core, and we also support python 3.8 here. However this change only effects python versions less than 3.8 which neither dbt-common nor dbt-core support