[pkg] require python >= 3.4
This avoid pip pulling a version that does not run on python2 when using a
python2 environment.
Since we already released some 3.27 releases in pypi, I think we should release
3.27.3 and remove releases 3.27.2, 3.27.1 and 3.27.0 from pypi.
Release notes
-------------
.. toctree::
:maxdepth: 1
3.27
3.26
3.25
3.24
3.23
3.22
3.21
3.20
3.19
3.18
3.17
3.16
3.15
3.14