add --version option to pybind11-config (#4526)

Without this, it's impossible to get feature parity between detection
mechanisms. Both the pkg-config file and the cmake config set their
versions, but the python probe script didn't provide an option for this.

So you could print the compiler flags for using it, but you could not
check what you got.

pybind11/__main__.py

diff --git a/pybind11/__main__.py b/pybind11/__main__.py
index e87b16b..180665c 100644
--- a/pybind11/__main__.py
+++ b/pybind11/__main__.py
@@ -4,6 +4,7 @@
 import sys
 import sysconfig
 
+from ._version import __version__
 from .commands import get_cmake_dir, get_include, get_pkgconfig_dir
 
 
@@ -26,6 +27,12 @@
 def main() -> None:
     parser = argparse.ArgumentParser()
     parser.add_argument(
+        "--version",
+        action="version",
+        version=__version__,
+        help="Print the version and exit.",
+    )
+    parser.add_argument(
         "--includes",
         action="store_true",
         help="Include flags for both pybind11 and Python headers.",