unix: upgrade Linux build environment from Debian Wheezy to Jessie

I am no longer able to run Debian Wheezy Docker images on my machine
because vsyscall support is no longer enabled after a recent distro
upgrade. This was a long time coming: vsyscall support is effectively
dead in modern Linux distros.

This commit upgrades our Linux build environment from Debian Wheezy
to Jessie (Debian 7 to 8). Debian Jessie doesn't rely on vsyscall
and people on modern distros should be able to use its Docker images.

The user-facing impact of this change is that glibc in the build
environment has been upgraded from 2.13 to 2.19 and glibc symbol
versions in produced binaries now require glibc >= 2.19 on the end-user
machine. This effectively changes the host machine requirements for
various Linux distros:

* Fedora 16 -> Fedora 21 (released December 2014)
* RHEL/CentOS 6 -> 7 (released June 2014)
* openSUSE 12.1 -> 13.2 (released November 2014)
* Debian 7 -> 8 (released April 2015)
* Ubuntu 12.04 -> 14.04 (released April 2014)

The new Linux distros are all sufficiently old that I think the upgrade
is tolerable.

Closes #6.

Author: indygreg

cpython-unix/base.Dockerfile

@@ -1,5 +1,5 @@
-# Debian Wheezy.
-FROM debian@sha256:37103c15605251b2e35b70a3214af626a55cff39abbaadccd01ff828ee7005e0
+# Debian Jessie.
+FROM debian@sha256:734728c8e411698485ae644fc988dad06f757565e292b5b85edc084befa37bbb
 MAINTAINER Gregory Szorc <[email protected]>
 
 RUN groupadd -g 1000 build && \
@@ -17,8 +17,8 @@ ENV HOME=/build \
 CMD ["/bin/bash", "--login"]
 WORKDIR '/build'
 
-RUN for s in debian_wheezy debian_wheezy-updates debian_wheezy-backports debian-security_wheezy/updates; do \
-      echo "deb http://snapshot.debian.org/archive/${s%_*}/20190321T212815Z/ ${s#*_} main"; \
+RUN for s in debian_jessie debian_jessie-updates debian-security_jessie/updates; do \
+      echo "deb http://snapshot.debian.org/archive/${s%_*}/20200510T203930Z/ ${s#*_} main"; \
     done > /etc/apt/sources.list && \
     ( echo 'quiet "true";'; \
       echo 'APT::Get::Assume-Yes "true";'; \