build/pkgs/python3: Remove libpng from dependencies, SAGE_SPKG_DEPCHECK

[mkoeppe]

python3 does not seem to depend at all on libpng, so we should remove it from the declared dependencies (and the SAGE_SPKG_DEPCHECK).

This will allow us to accept system python 3.6 on centos-7 (see #29033), as can be seen by using

  tox -e docker-centos-7-standard -- config.status

and probably also on conda-forge-ubuntu-minimal (where currently the python3 build fails, even after the upgrade to 3.9.1 in #30589, https://github.com/mkoeppe/sage/runs/1566872755).

CC: @kiwifb @jhpalmieri @isuruf

Component: packages: standard

Author: Matthias Koeppe

Branch/Commit: b739470

Reviewer: Isuru Fernando

Issue created by migration from https://trac.sagemath.org/ticket/31009

[mkoeppe]

Dependencies: #30559

[mkoeppe]

Branch: u/mkoeppe/build_pkgs_python3__remove_libpng_from_dependencies__sage_spkg_depcheck

[mkoeppe]

New commits:

dd3fc1e	build/pkgs/python3/spkg-configure.m4: Remove DEPCHECK on sqlite
efb9dcc	build/pkgs/sqlite/spkg-configure.m4: Set as not required if system python3 will be used
a39a9f1	build/pkgs/cryptominisat/dependencies: Remove unused dep on sqlite
362f6c9	build/pkgs/elliptic_curves/dependencies: Remove sqlite; the package only uses it through the sqlite python module
5ca8742	Merge tag '9.3.beta0' into t/30559/refine_python3_s_sage_spkg_depcheck
08e3369	sage.misc.sage_ostools: Link to sqlite only on Cygwin
4731840	Merge tag '9.3.beta1' into t/30559/refine_python3_s_sage_spkg_depcheck
ed2e3d4	Merge commit '4731840945a8598cbc97367e2b4ad60e6d9c6c7c' of git://trac.sagemath.org/sage into t/31009/build_pkgs_python3__remove_libpng_from_dependencies__sage_spkg_depcheck
b739470	build/pkgs/python3: Remove libpng from dependencies, SAGE_SPKG_DEPCHECK

[mkoeppe]

Commit: b739470

[mkoeppe]

Author: Matthias Koeppe

[mkoeppe]

Changed dependencies from #30559 to none

[isuruf]

Reviewer: Isuru Fernando

[mkoeppe]

comment:7

Thanks!

[vbraun]

Changed branch from u/mkoeppe/build_pkgs_python3__remove_libpng_from_dependencies__sage_spkg_depcheck to b739470