[Error] Remove deprecations in taichi/__init__.py in v1.6.0 (#7222)

Issue: #7189

### Brief Summary

python/taichi/__init__.py

@@ -59,33 +59,26 @@
      'lang.mesh._TetMesh')
 }
 
-if sys.version_info.minor < 7:
-    for name, alter in __deprecated_names__.items():
-        exec(f'{name} = {alter}')
-    for _origin, (_msg, _replace) in __customized_deprecations__.items():
-        exec(f'{_origin} = {_replace}')
-else:
 
-    def __getattr__(attr):
-        # There's no easy way to hook accessing attribute with function calls in python3.6.
-        # So let's skip it for now.
-        import warnings  # pylint: disable=C0415,W0621
-        if attr == 'cfg':
-            return None if lang.impl.get_runtime(
-            ).prog is None else lang.impl.current_cfg()
-        if attr in __deprecated_names__:
-            warnings.warn(
-                f'ti.{attr} is deprecated. Please use ti.{__deprecated_names__[attr]} instead.',
-                DeprecationWarning)
-            exec(f'{attr} = {__deprecated_names__[attr]}')
-            return locals()[attr]
-        if attr in __customized_deprecations__:
-            msg, fun = __customized_deprecations__[attr]
-            warnings.warn(f'ti.{attr} is deprecated. {msg}',
-                          DeprecationWarning)
-            exec(f'{attr} = {fun}')
-            return locals()[attr]
-        raise AttributeError(f"module '{__name__}' has no attribute '{attr}'")
+def __getattr__(attr):
+    import warnings  # pylint: disable=C0415,W0621
+    if attr == 'cfg':
+        return None if lang.impl.get_runtime(
+        ).prog is None else lang.impl.current_cfg()
+    if attr in __deprecated_names__:
+        warnings.warn(
+            f'ti.{attr} is deprecated, and it will be removed in Taichi v1.6.0. Please use ti.{__deprecated_names__[attr]} instead.',
+            DeprecationWarning)
+        exec(f'{attr} = {__deprecated_names__[attr]}')
+        return locals()[attr]
+    if attr in __customized_deprecations__:
+        msg, fun = __customized_deprecations__[attr]
+        warnings.warn(
+            f'ti.{attr} is deprecated, and it will be removed in Taichi v1.6.0. {msg}',
+            DeprecationWarning)
+        exec(f'{attr} = {fun}')
+        return locals()[attr]
+    raise AttributeError(f"module '{__name__}' has no attribute '{attr}'")
 
 
 __version__ = (_ti_core.get_version_major(), _ti_core.get_version_minor(),

tests/python/test_deprecation.py

@@ -135,3 +135,12 @@ def ker(tex: ti.types.rw_texture(num_dimensions=2, lod=0)):
             for i, j in ti.ndrange(n, n):
                 ret = ti.cast(1, ti.f32)
                 tex.store(ti.Vector([i, j]), ti.Vector([ret, 0.0, 0.0, 0.0]))
+
+
+@test_utils.test()
+def test_deprecation_in_taichi_init_py():
+    with pytest.warns(
+            DeprecationWarning,
+            match=
+            "ti.SOA is deprecated, and it will be removed in Taichi v1.6.0."):
+        ti.SOA