MAINT: require Cython 0.19

Earlier versions have memory leaks in fused types.

(cherry picked from commit 6cb8361b8f32bff3d3fffe24ac1e27bd85c1bff2)
DOC: add cython dependency to INSTALL

(cherry picked from commit 51b71456ac53790e9b0d909b48406d4cf2256954)
DOC: make more clear when Cython is required.

(cherry picked from commit fb15a61d48bd34811a8746c605729cf7290855a7)
DOC: bump Cython requirement in INSTALL.txt

(cherry picked from commit 7e1dddedaeda1e107b04691b6be9136db12c315c)