Revert "FIX: replace spurious 1.0E+12 by finfo(float).max in slsqp"

This reverts commit ccecd81bc4490b1cf71be85d31d838013d8fca7e.

Closes #3108.

(cherry picked from commit 1c04787ac5064421a61d5a042ce4916b28ace8ba)
DOC: Document how SLSP handles infinite values in bounds

(cherry picked from commit 03d2431c0c57fb4c446393ca5a19308d8dc1eca4)
TST: Add a test for SLSQP with bounds

(cherry picked from commit 6a0fa36ada20f5fe3d7295783f86cbb48f2e709c)
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)