Trac #29404: Fix build failures of pynac on macos with system python3

https://github.com/mkoeppe/sage/runs/531095486

pynac:
{{{
checking whether sage-python23 version is >= 2.7... yes
checking for sage-python23 version... 3.7
checking for sage-python23 platform... darwin
checking for sage-python23 script directory... ${prefix}/lib/python3.7
/site-packages
checking for sage-python23 extension module directory...
${exec_prefix}/lib/python3.7/site-packages
checking for style of include used by make... GNU
checking for gcc... gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables...
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking for gcc option to accept ISO C89... none needed
checking whether gcc understands -c and -o together... yes
checking dependency style of gcc... none
checking for python3.7... no
configure: error: Cannot find python3.7 in your system path
}}}

Fix in pynac/pynac#351, https://github.com/mkoep
pe/pynac/releases/tag/pynac-0.7.26.sage-2020-03-28

The "temporary upstream" tarball URL is in `checksums.ini`. To test the
update on this branch: Use `make SAGE_SPKG="sage-spkg -o" pynac-clean
pynac`; this will download the new upstream package automatically.

URL: https://trac.sagemath.org/29404
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

build/pkgs/pynac/checksums.ini

@@ -1,4 +1,5 @@
 tarball=pynac-VERSION.tar.bz2
-sha1=895950af30f818d4bbd55d6695c33ec63515fd5f
-md5=44f1f0a2c5feff7b3e30fa763ff7e2c0
-cksum=3238748787
+sha1=4913173e8bbb3d79bb4ee1faf631b154c9b8ef8c
+md5=b38234ffedc018e7f31217dcc2879035
+cksum=881228674
+upstream_url=https://github.com/mkoeppe/pynac/releases/download/pynac-VERSION/pynac-VERSION.tar.bz2

build/pkgs/pynac/package-version.txt

@@ -1 +1 @@
-0.7.26
+0.7.26.sage-2020-04-03