Declare PyQt5 through an optional compatibility extra

CHANGELOG.md

@@ -6,6 +6,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## To be released
 ### Added
+* Add a `pyqt5` extra for optional PyQt5 version compatibility constraints.
 * Add the `sip.SIP_ABI_VERSION` integer constant while retaining stubtest compatibility with older SIP versions.
 * [#192](https://github.com/python-qt-tools/PyQt5-stubs/pull/192) Add missing platform specific stubs:
   * Windows specific: QAxContainer.pyi QtWinExtras.pyi QtCore.QWinEventNotifier

README.md

@@ -22,10 +22,18 @@ appreciate any PRs or Issues that help making this stub-repository more reliable
 
 # Installation
 
-Simply install PyQt5-stubs with pip:
+Install the stubs with pip:
 
     $ pip install PyQt5-stubs
 
+To opt into enforcement of any declared PyQt5 version compatibility constraints,
+select the `pyqt5` extra:
+
+    $ pip install "PyQt5-stubs[pyqt5]"
+
+The extra currently declares no PyQt5 version bounds. Selecting it may also
+cause pip to install PyQt5 as a side effect of satisfying the dependency.
+
 Or clone the latest version from Github and install it via Python setuptools:
 
     $ git clone https://github.com/python-qt-tools/PyQt5-stubs

setup.py

@@ -51,6 +51,7 @@ def find_version(*file_paths):
     package_data={"PyQt5-stubs": ['*.pyi']},
     packages=["PyQt5-stubs"],
     extras_require={
+        "pyqt5": ["PyQt5"],
         "dev": ["mypy==0.930", "pytest<9", "pytest-xvfb"],
     },
     classifiers=[