python: Fix using overrides when not building PyGObject

Since 547570cd790f2b2e390edc1dfb5df4c7a33de45c we do not always build
PyGObject and our development environment is broken when trying to use
GStreamer python when built against system PyGObject with the following
error importing Gst in there:

```
12345678** (gst-plugin-scanner:710617): CRITICAL **: 11:45:02.343: can't find gi.repository.Gst
Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/gi/repository/__init__.py", line 23, in <module>
    from ..importer import DynamicImporter
  File "/usr/lib64/python3.9/site-packages/gi/importer.py", line 33, in <module>
    from .overrides import load_overrides
ImportError: cannot import name 'load_overrides' from 'gi.overrides' (/var/home/thiblahute/devel/gstreamer/gstreamer/subprojects/gst-editing-services/bindings/python/gi/overrides/__init__.py)
Factory Details:
```

The approach to fixing it is to implement override `gi` in
`gst-python/gi/` which we add to `PYTHONPATH`) and in there reset the
`gi` module to the right place and we get overrides from paths from
`_GI_OVERRIDES_PATH` we set in `gst-env.py` which points to all the
overrides that will be installed.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1155>

2 files changed, 39 insertions, 0 deletions

diff --git a/tests/python/meson.build b/tests/python/meson.build
new file mode 100644
index 0000000000..b391044b7e
--- /dev/null
+++ b/tests/python/meson.build
@@ -0,0 +1,19 @@
+gst_python = subproject('gst-python', required: false)
+gir = find_program('g-ir-scanner', required : get_option('introspection'))
+if not gst_python.found() or not gir.found()
+    message('Not running python devenv tests: gst_python: @0@ gir: @1@'.format(gst_python.found(), gir.found()))
+    subdir_done()
+endif
+
+root_rel = '../..'
+python = import('python').find_installation()
+
+if run_command(python, '-c', 'import gi').returncode() != 0
+    message('PyGObject not found, not running PyGObject tests')
+    subdir_done()
+endif
+
+test('python-overrides-devenv', setenv, args: ['--builddir=@0@'.format(meson.build_root()),
+              '--gstbuilddir=@0@'.format(meson.current_build_dir() / '..' / '..'),
+              '--srcdir=@0@'.format(meson.source_root()),
+              meson.current_source_dir() / 'python-devenv-overrides.py'])
\ No newline at end of file
diff --git a/tests/python/python-devenv-overrides.py b/tests/python/python-devenv-overrides.py
new file mode 100755
index 0000000000..0c099118ea
--- /dev/null
+++ b/tests/python/python-devenv-overrides.py
@@ -0,0 +1,20 @@
+#!/usr/bin/python3
+import unittest
+from pathlib import Path
+from unittest import TestCase
+from gi.repository import Gst
+
+
+class TestBin(TestCase):
+    def test_overrides(self):
+        from gi.overrides import Gst
+        self.assertEqual(Path(Gst.__file__), Path(__file__).parents[2] / "subprojects/gst-python/gi/overrides/Gst.py")
+
+    def simple_functional_test(self):
+        Gst.init(None)
+        self.assertEqual(Gst.ElementFactory.make("bin", None).sinkpads, [])
+        self.assertEqual(float(Gst.Fraction(1, 2)), 0.5)
+
+
+if __name__ == "__main__":
+    unittest.main()