Drop CentOS6 compatibility
The buildbot docker images for x86_64 and x86_32 are based on manylinux2010, which is in turn based on CentOS6, which is EOL Nov 2020. We should move to manylinux2014-based docker images. This might allow embedding more updated versions of lzma and gdbm (in build_cffi_imports.py
), which were excluded since they do not compile on the buildbot
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