Probability fractions not reduced in Python versions anterior to 3.9

[Pierre Denis]

When Lea is installed in Python versions up to 3.8, the probability fractions are not always reduced to the smallest common, denominator:

lea.event("1/3").times(2)
# -> 0 : 36/81
#    1 : 36/81
#    2 :  9/81

although, we could expect

# -> 0 : 4/9
#    1 : 4/9
#    2 : 1/9

as it is the case for Python 3.9+.

[Pierre Denis]

Fix 'lcm' method (refs #84)

→ <<cset 3d9782c3e5a9>>

[Pierre Denis]

Fix 'lcm' function on Lea 3 (refs #84, #85)

→ <<cset c04ed68004d3>>

[Pierre Denis]

Fix 'lcm' function on Lea 3 (refs #84, #85)

→ <<cset 1c8574782589>>

[Pierre Denis]

Fix 'lcm' function on Lea 3 (refs #84, #85)

→ <<cset ea5971ee5962>>

[Pierre Denis]

Fix 'lcm' function on Lea 4 for Python 3.8 (refs #84)

→ <<cset e5b3aed29cd7>>