docs: Fix duplicate label, undefined ref, and strikethrough warnings

why: Sphinx warns about duplicate labels, undefined references, and
strikethrough syntax not supported in non-HTML output.

what:
- Remove duplicate (internals)= label from about.md
- Add (setting_a_tmux_configuration)= label and fix reference in pytest-plugin/index.md
- Replace ~~strikethrough~~ with italicized note in CHANGES

Author: tony

CHANGES

@@ -1581,10 +1581,8 @@ _Maintenance only, no bug fixes or new features_
 
 - Python 3.7 and 3.8 returns in 0.12.0
 
-  ~~Final python 3.7 and 3.8 release~~
-
-  ~~Fixes and security updates will go to
-  [`v0.11.x`](https://github.com/tmux-python/libtmux/tree/v0.11.x)~~
+  *Note: This was not the final Python 3.7 and 3.8 release as originally stated.
+  Python 3.7 and 3.8 support was extended in 0.12.0.*
 
 - Internal: Use new separator to split `tmux(1)` formatting information (#289,
   #343)

docs/about.md

@@ -1,5 +1,3 @@
-(internals)=
-
 (about)=
 
 # About

docs/pytest-plugin/index.md

@@ -59,6 +59,8 @@ These fixtures are automatically used when the plugin is enabled and `pytest` is
 
   These are set to ensure panes and windows can be reliably referenced and asserted.
 
+(setting_a_tmux_configuration)=
+
 ## Setting a tmux configuration
 
 If you would like {func}`session fixture <libtmux.pytest_plugin.session>` to automatically use a configuration, you have a few
@@ -108,7 +110,7 @@ def test_something(TestServer):
     assert server.is_alive()
 ```
 
-You can also use it with custom configurations, similar to the {ref}`server fixture <Setting a tmux configuration>`:
+You can also use it with custom configurations, similar to the {ref}`server fixture <setting_a_tmux_configuration>`:
 
 ```python
 def test_with_config(TestServer, tmp_path):