Use floats for signed zeros in the real part of repr(complex)

>>> complex(-0.0, 1)  # was (-0+1j)
(-0.0+1j)

This doesn't break complex(str(x)) == x invariant as we
had support for parsing signed 0.0's here before.

Lib/test/test_complex.py

@@ -835,14 +835,14 @@ def test(v, expected, test_fn=self.assertEqual):
             test_fn(str(v), expected)
 
         test(complex(0., 1.),   "1j")
-        test(complex(-0., 1.),  "(-0+1j)")
+        test(complex(-0., 1.),  "(-0.0+1j)")
         test(complex(0., -1.),  "-1j")
-        test(complex(-0., -1.), "(-0-1j)")
+        test(complex(-0., -1.), "(-0.0-1j)")
 
         test(complex(0., 0.),   "0j")
         test(complex(0., -0.),  "-0j")
-        test(complex(-0., 0.),  "(-0+0j)")
-        test(complex(-0., -0.), "(-0-0j)")
+        test(complex(-0., 0.),  "(-0.0+0j)")
+        test(complex(-0., -0.), "(-0.0-0j)")
 
     def test_pos(self):
         self.assertEqual(+(1+6j), 1+6j)

Objects/complexobject.c

@@ -476,8 +476,8 @@ complex_repr(PyComplexObject *v)
     } else {
         /* Format imaginary part with sign, real part without. Include
            parens in the result. */
-        pre = PyOS_double_to_string(v->cval.real, format_code,
-                                    precision, 0, NULL);
+        pre = PyOS_double_to_string(v->cval.real, format_code, precision,
+                                    v->cval.real? 0 : Py_DTSF_ADD_DOT_0, NULL);
         if (!pre) {
             PyErr_NoMemory();
             goto done;

Python/formatter_unicode.c

@@ -1290,8 +1290,13 @@ format_complex_internal(PyObject *value,
     /* Cast "type", because if we're in unicode we need to pass an
        8-bit char. This is safe, because we've restricted what "type"
        can be. */
-    re_buf = PyOS_double_to_string(re, (char)type, precision, flags,
-                                   &re_float_type);
+    if (re == 0.0 && copysign(1.0, re) == -1.0)
+        re_buf = PyOS_double_to_string(re, (char)type, precision,
+                                       flags | Py_DTSF_ADD_DOT_0,
+                                       &re_float_type);
+    else
+        re_buf = PyOS_double_to_string(re, (char)type, precision, flags,
+                                       &re_float_type);
     if (re_buf == NULL)
         goto done;
     im_buf = PyOS_double_to_string(im, (char)type, precision, flags,