bpo-30464: Fix comment in gammavariate

[leodema]

A comment in the gammavariate function in random.py make it seems that when alpha is 1 it's equivalent on calling expovariate(1), but it should be expovariate(1/beta)

also discussed in:
https://mail.python.org/pipermail/python-bugs-list/2001-January/003752.html

where Ivan Frohne says:

Now it happens that a gamma-distributed random variable with parameters A =
1 and B has the (much simpler) exponential distribution with density
function

g(x; 1, B) = exp(-x/B) / B.

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[mention-bot]

@leodema, thanks for your PR! By analyzing the history of the files in this pull request, we identified @rhettinger, @tim-one and @tiran to be potential reviewers.

[mdickinson]

LGTM. I think this can be merged as soon as the CLA detail is sorted out. The implementation change that @rhettinger refers to in the issue discussion can easily happen in a separate PR.

[Mariatta]

@leodema I don't think the bpo number 46895 is a valid one.
Can you please check? Thanks.

[leodema]

sorry guys! not sure where I got that bpo, thanks @Mariatta and @serhiy-storchaka for fixing it

[csabella]

Hi @leodema - @rhettinger commented on the bpo on 5/25 to request additional changes, please take a look when you get a chance. Thanks!

[leodema]

Hi @csabella I'll be happy to but I thought it would be part of a separate PR as @mdickinson suggested in a later post, but ok I will do it here, thanks.

[csabella]

@leodema I apologize. I didn't see that earlier comment. You can ignore what I said earlier.

[mdickinson]

Let's merge this. It's an obvious fix that's mathematically correct and that Raymond has already approved in the issue discussion.