Drop some workarounds for GraalPy

See #232

Author: skirpichev

tests/test_mpz.py

@@ -517,8 +517,6 @@ def test_divmod_bulk(x, y):
         with pytest.raises(ZeroDivisionError):
             x % my
         return
-    if y < 0 and platform.python_implementation() == "GraalVM":
-        return  # issue oracle/graalpython#534
     r = x // y
     assert mx // my == r
     assert mx // y == r
@@ -718,8 +716,6 @@ def test_power_errors():
         pow(mpz(10**1000), 1j)
     with pytest.raises(TypeError):
         pow(object(), mpz(321))
-    if platform.python_implementation() == "GraalVM":
-        return  # issue oracle/graalpython#551
     with pytest.raises(OverflowError):
         pow(mpz(2), mpz(1<<64))
     if BITCNT_MAX < (1<<64) - 1: