Stop testing against Twisted 15

Since it requires the now removed from setuptools pkg_resources and
last release is more than 10 years old.

Author: xrmx

.ci/.matrix_exclude.yml

@@ -353,32 +353,24 @@ exclude:
     FRAMEWORK: twisted-17
   - VERSION: python-3.11
     FRAMEWORK: twisted-16
-  - VERSION: python-3.11
-    FRAMEWORK: twisted-15
   - VERSION: python-3.12
     FRAMEWORK: twisted-18
   - VERSION: python-3.12
     FRAMEWORK: twisted-17
   - VERSION: python-3.12
     FRAMEWORK: twisted-16
-  - VERSION: python-3.12
-    FRAMEWORK: twisted-15
   - VERSION: python-3.13
     FRAMEWORK: twisted-18
   - VERSION: python-3.13
     FRAMEWORK: twisted-17
   - VERSION: python-3.13
     FRAMEWORK: twisted-16
-  - VERSION: python-3.13
-    FRAMEWORK: twisted-15
   - VERSION: python-3.14
     FRAMEWORK: twisted-18
   - VERSION: python-3.14
     FRAMEWORK: twisted-17
   - VERSION: python-3.14
     FRAMEWORK: twisted-16
-  - VERSION: python-3.14
-    FRAMEWORK: twisted-15
   # pylibmc
   - VERSION: python-3.11
     FRAMEWORK: pylibmc-1.4

.ci/.matrix_framework_full.yml

@@ -36,7 +36,6 @@ FRAMEWORK:
   - twisted-18
   - twisted-17
   - twisted-16
-  - twisted-15
   - requests-newest
   - boto3-1.5
   - boto3-1.6