Declare `typing_extensions` dependency

[jwodder]

Use of typing_extensions was reintroduced to the code in commit 0d381179 (#203), but typing_extensions was not added to the runtime requirements. This PR fixes that.

[codecov[bot]]

Codecov Report

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

Project coverage is 97.74%. Comparing base (2643846) to head (b20ebee).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #248 +/- ## ======================================= Coverage 97.74% 97.74% ======================================= Files 16 16 Lines 1727 1727 ======================================= Hits 1688 1688 Misses 39 39 ``` | [Flag](https://app.codecov.io/gh/dandi/dandi-schema/pull/248/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/dandi/dandi-schema/pull/248/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi) | `97.74% <ø> (ø)` | | 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=dandi#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[yarikoptic]

Thank you!