build: relax typeguard constraints following TensorFlow release

[alexander-held]

To avoid a typing_extension clash caused by TensorFlow, #429 restricted the typeguard version listed in the test setup extra. Now that an updated TensorFlow version is available, which fixes this clash, the typeguard version restriction can be relaxed again.

The typing-extensions cap introduced in tensorflow-probability has been fixed, but the versions with the fix are Python 3.9+ and therefore cause issues in the CI for 3.8. To avoid this, upgrade the typing_extensions version in the relevant Python 3.8 CI.

resolves #428

* relax version constraint on typeguard as latest TensorFlow release no longer causes dependency clash
* upgrade typing-extensions version in Python 3.8 CI to circumvent dependency clash

[codecov[bot]]

Codecov Report

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

Comparison is base (d5e2241) 100.00% compared to head (d2517cb) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #440 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 22 22 Lines 2082 2082 Branches 340 340 ========================================= Hits 2082 2082 ```

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