Increment `PYBIND11_VERSION_PATCH`

QA:

```
$ nox -s tests_packaging
nox > Running session tests_packaging
nox > Creating virtual environment (virtualenv) using python3 in .nox/tests_packaging
nox > python -m pip install -r tests/requirements.txt
nox > pytest tests/extra_python_package
=========================================================== test session starts ============================================================
platform darwin -- Python 3.10.2, pytest-7.4.4, pluggy-1.5.0
rootdir: /Users/rgrossekunst/forked/pybind11/tests/extra_python_package
configfile: pytest.ini
plugins: timeout-2.3.1
collected 4 items

tests/extra_python_package/test_files.py ....                                                                                        [100%]

============================================================ 4 passed in 22.82s ============================================================
nox > Session tests_packaging was successful.
```

include/pybind11/detail/common.h

@@ -11,11 +11,11 @@
 
 #define PYBIND11_VERSION_MAJOR 2
 #define PYBIND11_VERSION_MINOR 12
-#define PYBIND11_VERSION_PATCH 0
+#define PYBIND11_VERSION_PATCH 1
 
 // Similar to Python's convention: https://docs.python.org/3/c-api/apiabiversion.html
 // Additional convention: 0xD = dev
-#define PYBIND11_VERSION_HEX 0x020C0000
+#define PYBIND11_VERSION_HEX 0x020C0100
 
 // Define some generic pybind11 helper macros for warning management.
 //

pybind11/_version.py

@@ -8,5 +8,5 @@ def _to_int(s: str) -> Union[int, str]:
         return s
 
 
-__version__ = "2.12.0"
+__version__ = "2.12.1"
 version_info = tuple(_to_int(s) for s in __version__.split("."))