Support expression as message in assert

[Shaikh-Ubaid]

towards #2061 (comment), #2054

[certik]

Shouldn't the second argument be a string? According to the docs: https://docs.python.org/3/reference/simple_stmts.html#grammar-token-python-grammar-assert_stmt, it says that assert expression1, expression2 is equivalent to:

if __debug__:
    if not expression1: raise AssertionError(expression2)

And I thought you put in a string into an exception constructor.

In any case, using strings is the most common I think, so I would at least test that, to make sure strings work.

[Shaikh-Ubaid]

It seems expressions are also supported (since the added tests in this PR with expressions as assert message work with CPython).

[Shaikh-Ubaid]

I updated the test expr_18 in TEST: Add test for expr as assert message to print a string on assert failure.

[Shaikh-Ubaid]

I updated the test expr_17 to have a string message with assert.

[certik]

Otherwise I think this looks good.

[certik]

I think that looks good, thanks!