Update to PyO3 0.16, num-bigint 0.4, num-rational 0.4

[LegionMammal978]

Fixes #2. The updates to num-bigint and num-rational are necessary to match the versions expected by PyO3. Note that this bumps the required Python version from 3.5 to 3.7 (released 2018-06-27) and the MSRV from nightly-2020-01-21 to 1.40.0 (released 2019-12-19).

[programmerjake]

Thanks for the contribution!

This broke the python tests. PyNumberProtocol appears to have automatically implemented the __radd__-like functions in terms of the __add__-like functions, switching to just plain methods broke that.

test commands:

pip3 install .
python3 -m unittest

unittest output:

.E...E......E.E..E...
======================================================================
ERROR: test_add (tests.test_real_algebraic_number.TestRealAlgebraicNumber)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jacob/projects/algebraics/tests/test_real_algebraic_number.py", line 188, in test_add
    self.assertEqual(1 + RealAlgebraicNumber(2), 3)
TypeError: unsupported operand type(s) for +: 'int' and 'algebraics.RealAlgebraicNumber'

======================================================================
ERROR: test_div (tests.test_real_algebraic_number.TestRealAlgebraicNumber)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jacob/projects/algebraics/tests/test_real_algebraic_number.py", line 204, in test_div
    self.assertEqual(1 / RealAlgebraicNumber(2),
TypeError: unsupported operand type(s) for /: 'int' and 'algebraics.RealAlgebraicNumber'

======================================================================
ERROR: test_mul (tests.test_real_algebraic_number.TestRealAlgebraicNumber)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jacob/projects/algebraics/tests/test_real_algebraic_number.py", line 198, in test_mul
    self.assertEqual(1 * RealAlgebraicNumber(2), 2)
TypeError: unsupported operand type(s) for *: 'int' and 'algebraics.RealAlgebraicNumber'

======================================================================
ERROR: test_pow (tests.test_real_algebraic_number.TestRealAlgebraicNumber)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jacob/projects/algebraics/tests/test_real_algebraic_number.py", line 219, in test_pow
    self.assertEqual(1 ** RealAlgebraicNumber(2), 1)
TypeError: unsupported operand type(s) for ** or pow(): 'int' and 'algebraics.RealAlgebraicNumber'

======================================================================
ERROR: test_sub (tests.test_real_algebraic_number.TestRealAlgebraicNumber)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jacob/projects/algebraics/tests/test_real_algebraic_number.py", line 193, in test_sub
    self.assertEqual(1 - RealAlgebraicNumber(2), -1)
TypeError: unsupported operand type(s) for -: 'int' and 'algebraics.RealAlgebraicNumber'

----------------------------------------------------------------------
Ran 21 tests in 0.014s

FAILED (errors=5)

[LegionMammal978]

Just got the __r*__ methods working with a From<&'_ PyCell<RealAlgebraicNumberPy2>> for SharedNumber impl. I cleaned up the other impls while I was at it, and renamed RealAlgebraicNumberWrapper to SharedNumber to reduce clutter since it appears over 50 times in the file. Also, the Arc::from_mut() in try_arithmetic_helper was useless, since the RealAlgebraicNumberPy2 always holds its own strong reference, so I replaced it with a standard return.

[programmerjake]

Thank you very much!

[programmerjake]

I published v0.3.0 to PyPI and crates.io.

cc @rotu