Install python-gdb.py.

The script in `Tools/gdb/libpython.py` has helper functions for
debugging Python processes under gdb. When installed to `{exe}-gdb.py`
it will automatically be loaded by gdb assuming it is in the safe path.

gdb resolves all symlinks, so having the single script is sufficient
regardless of how Python is invoked.

Since it is usually only included as part of a dbg/dev package, this
file is intentionally excluded from install by the CPython build
scripts. Like the packaging with EPEL/Debian, we opt to manually copy
it after the build/install.

To ensure this file gets picked up it is enough for users to put this in
their `~/.gdbinit`:

```
add-auto-load-safe-path ~/.pyenv
```

Fixes #1190.

plugins/python-build/bin/python-build

@@ -1813,6 +1813,16 @@ build_package_verify_py38() {
   build_package_verify_py37 "$1" "${2:-3.8}"
 }
 
+# Copy Tools/gdb/libpython.py to pythonX.Y-gdb.py (#1190)
+build_package_copy_python_gdb() {
+  if [ -e "$BUILD_PATH/$1/Tools/gdb/libpython.py" ]; then
+    local version_re='-([0-9]\.[0-9]+)'
+    [[ "$1" =~ $version_re ]]
+    local python_bin="$PREFIX_PATH/bin/python${BASH_REMATCH[1]}"
+    cp "$BUILD_PATH/$1/Tools/gdb/libpython.py" "$python_bin-gdb.py"
+  fi
+}
+
 build_package_ez_setup() {
   local ez_setup="ez_setup.py"
   rm -f "${ez_setup}"