build(deps): update testfixtures requirement

Updates the requirements on [testfixtures](https://github.com/Simplistix/testfixtures) to permit the latest version.
- [Changelog](https://github.com/simplistix/testfixtures/blob/main/CHANGELOG.rst)
- [Commits](simplistix/testfixtures@8.3.0...12.0.0)

---
updated-dependencies:
- dependency-name: testfixtures
  dependency-version: 12.0.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

setup.py

@@ -35,6 +35,6 @@
         'pytest>=7.2.2, <9.1.0',
         'coverage>=7.2.2, <7.14.0',
         'pytest-cov>=4.0, <7.1',
-        'testfixtures>=8.3.0, <= 10.0.0'
+        'testfixtures>= 8.3.0, <= 12.0.0'
     ]
 )

test-requirements.txt

@@ -1,4 +1,4 @@
 pytest>=7.2.2, <9.1.0
 pytest-mock~=3.14.0
 coverage>=7.2.2, <7.14.0
-testfixtures>=8.3.0, <= 10.0.0
+testfixtures<= 12.0.0, >= 12.0.0