TST: Run the new univariate tests without matplotlib

[bashtage]

Branch a90-fix-tests-without-matplotlib, one commit on main 80d3688.

The arch-wheels trial run (after arch-wheels#29) failed on every platform and
Python version. One cause was the 7 dtype tests that #957 has since fixed. The
other is still on main: three test modules that are new in this release import
matplotlib.pyplot at module level:

• arch/tests/univariate/test_positions_and_alias.py
• arch/tests/univariate/test_string_options.py
• arch/tests/univariate/test_validation_base.py

The arch-wheels test environment does not install matplotlib, so these modules
fail to be collected (ModuleNotFoundError: No module named 'matplotlib').
arch's own CI always installs matplotlib, so it never showed there.

Fix (tests only), following the existing tests (test_hedgehog_plot.py,
test_mean.py):

• import matplotlib.pyplot only if arch.compat.matplotlib.HAS_MATPLOTLIB;
• skip the plot and hedgehog_plot tests and table rows without it;
• close figures only when matplotlib is available.

With matplotlib blocked (the three modules, one process): 854 passed, 133
skipped, 0 failed (before: collection errors, and 349 failures once the import
was guarded but the plot tests were not skipped). With matplotlib: fast suite
26,955 passed.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 73.68421% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.95%. Comparing base (80d3688) to head (310a5b1).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
arch/tests/univariate/test_validation_base.py	62.50%	1 Missing and 2 partials ⚠️
arch/tests/univariate/test_positions_and_alias.py	75.00%	0 Missing and 1 partial ⚠️
arch/tests/univariate/test_string_options.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #960      +/-   ##
==========================================
- Coverage   98.97%   98.95%   -0.02%     
==========================================
  Files         136      136              
  Lines       30028    30043      +15     
  Branches     2327     2331       +4     
==========================================
+ Hits        29719    29729      +10     
- Misses        255      256       +1     
- Partials       54       58       +4     

Flag	Coverage Δ
adder	98.89% <73.68%> (-0.02%)	⬇️
subtractor	98.89% <73.68%> (-0.02%)	⬇️

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