Bytes constructor test change compatible with PR #2380 (#2390)

* Rolling back PR #2340 change to tests/test_pytypes.py (only this one file).

The two other files changed with PR #2340 are not affected by this partial rollback.

This partial rollback enables cherry-picking a commit from PR #2380.

* test_constructors() fix for Python 2.

Preparation for changing `pybind11::str` to only hold `PyUnicodeObject` (NOT also `bytes`).

Currently test_constructors passes with Python 2 only because `pybind11::str` can also hold a Python 2 `PyStringObject` (or the equivalent `PyBytesObject` in Python 3). Changing the test to exercise conversions for `PyUnicodeObject` makes it consistent between Python 2 and 3, and removes this small obstacle to the planned `pybind11::str` change.

Tests for `bytes` conversions will be added separately.

* Adding test_constructors test for bytes, on top of cherry-picked commit from PR #2380.

tests/test_pytypes.py

@@ -198,19 +198,24 @@ def test_constructors():
     assert m.default_constructors() == expected
 
     data = {
-        "bytes": b'41',  # Currently no supported or working conversions.
-        "str": 42,
-        "bool": "Not empty",
-        "int": "42",
-        "float": "+1e3",
-        "tuple": range(3),
-        "list": range(3),
-        "dict": [("two", 2), ("one", 1), ("three", 3)],
-        "set": [4, 4, 5, 6, 6, 6],
-        "memoryview": b'abc'
+        bytes: b'41',  # Currently no supported or working conversions.
+        str: 42,
+        bool: "Not empty",
+        int: "42",
+        float: "+1e3",
+        tuple: range(3),
+        list: range(3),
+        dict: [("two", 2), ("one", 1), ("three", 3)],
+        set: [4, 4, 5, 6, 6, 6],
+        memoryview: b'abc'
     }
-    inputs = {k: v for k, v in data.items()}
-    expected = {k: eval(k)(v) for k, v in data.items()}
+    inputs = {k.__name__: v for k, v in data.items()}
+    expected = {k.__name__: k(v) for k, v in data.items()}
+    if str is bytes:  # Similar to the above. See comments above.
+        inputs["bytes"] = b'41'
+        inputs["str"] = 42
+        expected["bytes"] = b'41'
+        expected["str"] = u"42"
 
     assert m.converting_constructors(inputs) == expected
     assert m.cast_functions(inputs) == expected