python3-gmpy2: restore package, update to 2.1.0rc1

Upstream 2.1.0rc1 builds on python3.10, last official version does not.

This package is used by python3-mpmath for multiprecission arithmetic.
If not installed, mpmath defaults to python bigints which are very slow.

For example: compute 1M digits of pi using sympy:

$ isympy	# without python3-gmpy2
In [1]: %time a=pi.n(1_000_000)
CPU times: user 49.4 s, sys: 2.96 ms, total: 49.4 s
Wall time: 49.4 s

vs.

$ isympy	# with python3-gmpy2
In [1]: %time a=pi.n(1_000_000)
CPU times: user 632 ms, sys: 5.99 ms, total: 638 ms
Wall time: 639 ms

Note that just installing python3-gmpy2 brings this improvement.
Arguably python3-gmpy2 could be made a dependency of python3-mpmath.

This reverts commit f91cea99dd.

Also:
 - fix hostmakedepends
 - make tests work
 - adopt

srcpkgs/python3-gmpy2/template

@@ -0,0 +1,20 @@
+# Template file for 'python3-gmpy2'
+pkgname=python3-gmpy2
+version=2.1.0rc1
+revision=1
+wrksrc="gmpy2-${version}"
+build_style=python3-module
+hostmakedepends="python3-setuptools"
+makedepends="python3-devel gmp-devel mpfr-devel libmpc-devel"
+depends="python3"
+short_desc="Python3 interface to GMP, MPFR and MPC libraries"
+maintainer="Gonzalo Tornaría <tornaria@cmat.edu.uy>"
+license="LGPL-3.0-or-later"
+homepage="https://github.com/aleaxit/gmpy"
+distfiles="${PYPI_SITE}/g/gmpy2/gmpy2-${version}.tar.gz"
+checksum=86cb6d8e5837560c32c706d48d6ed25676be6b3c79e6aa5d245965b9e99231b9
+
+do_check() {
+	PYTHONPATH="$(cd build/lib* && pwd)" \
+		python3 test/runtests.py
+}

srcpkgs/python3-gmpy2/update

@@ -0,0 +1 @@
+ignore="*a[1-9] *b[1-9] *rc[1-9]"