PyPy: Fixes a segfault + add tests

[misl6]

• Add test rules for PyPy
• Fixed a test that may fail on PyPy (not on CI as python is available as an alias)
• Disabled tests for x86 on PyPy
• Removes usage (was intended as a temp fix) of PyUnicode_DecodeUTF16, which was segfaulting on PyPy (Fixes on Cython have been introduced here: cython/cython@5c9b32c)

[misl6]

@tshirtman , maybe you want to give it a look before the merge?

[tshirtman]

LGTM :)
Edit: we don't currently set a minimum version of cython in the pyproject.toml, maybe it's a good time to do so, asking for the version with that fix or newer?

[misl6]

LGTM :)
Edit: we don't currently set a minimum version of cython in the pyproject.toml, maybe it's a good time to do so, asking for the version with that fix or newer?

Good point! I guess is better to ask for Cython>=0.29.30 (as is the latest available, it contains the needed fixes, and seems to work great).

BTW, I see some improvements (regarding Cython version) that could be made to the CI , pyproject.toml and setup.py. But, may go outside of the scope of this PR, so looks cleaner to submit a subsequent PR (seems easy and should land in a couple of hours). (I will refer to this PR, so we can keep track of it)