bpo-39479:Add math.lcm() function: Least Common Multiple (#18547)

* Update math.rst

* Update math.rst

* updated whats new

* Update test_math.py

* Update mathmodule.c

* Update mathmodule.c.h

* Update ACKS

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* Update 3.9.rst

* Update 2020-02-18-12-37-16.bpo-39479.j3UcCq.rst

* Update math.rst

* Update 2020-02-18-12-37-16.bpo-39479.j3UcCq.rst

* Update test_math.py

* Update ACKS

* Update mathmodule.c.h

* Update mathmodule.c

* Update mathmodule.c.h

* Update mathmodule.c.h

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