Improve code style (for Lea 4)

[Pierre Denis]

Lea 3 code has several deviations from PEP 8 coding style.

Several are quite systematic: three simple quotes for docstings, and missing whitespace after comma. These can be easily spotted using pylint or IDE (e.g. PyCharm).

These two abovementioned systematic deviations shall be fixed.

Most ot other deviations should be fixed also (to be analyzed).

These fixes should preferably be made after #71.

[Pierre Denis]

Reformat code to fix deviations from PEP 8 (refs #74)

→ <<cset 9fe1b09bfdfc>>

[Pierre Denis]

Remove support to Python versions strictly less than v3.5; correct code style, typos (refs #73, #74)

→ <<cset 4dd8d171515e>>

[Pierre Denis]

Minor code clean-up (refs #74)

→ <<cset a28d51effb32>>