Drop Python 3.9 support and add DOLFINx v0.8 support

[matt-graham]

Adds DOLFINx v0.8 to test matrix and updates usages of DOLFINx API to ensure compatibility across versions v0.6 through v0.8.

Also drops Python 3.9 support on the basis this is now > 3 years old and out of both NEP29 and SPEC0 recommended support windows, and our test matrix is getting a bit unwieldy.

Also updates our type hints to use | in place of explicit Union or Optional where appropriate, and adds strict=True argument to a call to zip.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.9%. Comparing base (ad756b7) to head (c57e835).

Files	Patch %	Lines
src/dxh.py	92.9%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #22 +/- ## ======================================= + Coverage 96.3% 96.9% +0.7% ======================================= Files 1 1 Lines 186 191 +5 Branches 37 38 +1 ======================================= + Hits 179 185 +6 + Misses 5 4 -1 Partials 2 2 ```

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[matt-graham]

@samcunliffe not sure if you have bandwidth to review this at the moment - no problem if you want to leave it till you're on project in August! From meeting last week it seems that dxh is getting used 🎉 but lack of DOLFINx v0.8 support was flagged as a pain.

The codecov/patch fail I think is just because codecov doesn't seem to hit bits of code in if TYPECHECKING blocks.