Make all classes with the same instance size derive from a common base

In order to fully satisfy Python's inheritance type layout requirements,
all types should have a common 'solid' base. A solid base is one which
has the same instance size as the derived type (not counting the space
required for the optional `dict_ptr` and `weakrefs_ptr`). Thus, `object`
does not qualify as a solid base for pybind11 types and this can lead to
issues with multiple inheritance.

To get around this, new base types are created: one per unique instance
size. There is going to be very few of these bases. They ensure Python's
MRO checks will pass when multiple bases are involved.

tests/test_multiple_inheritance.py

@@ -1,3 +1,6 @@
+import pytest
+
+
 def test_multiple_inheritance_cpp():
     from pybind11_tests import MIType
 
@@ -49,6 +52,17 @@ def test_multiple_inheritance_mix2():
     assert mt.bar() == 4
 
 
+def test_multiple_inheritance_error():
+    """Inheriting from multiple C++ bases in Python is not supported"""
+    from pybind11_tests import Base1, Base2
+
+    with pytest.raises(TypeError) as excinfo:
+        # noinspection PyUnusedLocal
+        class MI(Base1, Base2):
+            pass
+    assert "Can't inherit from multiple C++ classes in Python" in str(excinfo.value)
+
+
 def test_multiple_inheritance_virtbase():
     from pybind11_tests import Base12a, bar_base2a, bar_base2a_sharedptr
 
@@ -60,3 +74,38 @@ def test_multiple_inheritance_virtbase():
     assert mt.bar() == 4
     assert bar_base2a(mt) == 4
     assert bar_base2a_sharedptr(mt) == 4
+
+
+def test_mi_static_properties():
+    """Mixing bases with and without static properties should be possible
+     and the result should be independent of base definition order"""
+    from pybind11_tests import mi
+
+    for d in (mi.VanillaStaticMix1(), mi.VanillaStaticMix2()):
+        assert d.vanilla() == "Vanilla"
+        assert d.static_func1() == "WithStatic1"
+        assert d.static_func2() == "WithStatic2"
+        assert d.static_func() == d.__class__.__name__
+
+        mi.WithStatic1.static_value1 = 1
+        mi.WithStatic2.static_value2 = 2
+        assert d.static_value1 == 1
+        assert d.static_value2 == 2
+        assert d.static_value == 12
+
+        d.static_value1 = 0
+        assert d.static_value1 == 0
+        d.static_value2 = 0
+        assert d.static_value2 == 0
+        d.static_value = 0
+        assert d.static_value == 0
+
+
+@pytest.unsupported_on_pypy
+def test_mi_dynamic_attributes():
+    """Mixing bases with and without dynamic attribute support"""
+    from pybind11_tests import mi
+
+    for d in (mi.VanillaDictMix1(), mi.VanillaDictMix2()):
+        d.dynamic = 1
+        assert d.dynamic == 1