Fix quantiles() compatibility issue

[PulpCattel]

Fixes #1340

statistics.quantiles() is new in Python 3.8, replace it with custom implementation to preserve compatibility with older Python versions.

Tested a bit with:

from statistics import quantiles
from random import random, randint

def get_percentiles(data):
    n = len(data)
    if n < 2:
        raise ValueError("Not enough data, at least two data points required")
    data = sorted(data)
    m = n - 1
    result = []
    for i in range(1, 100):
        j, delta = divmod(i * m, 100)
        interpolated = (data[j] * (100 - delta) + data[j + 1] * delta) / 100
        result.append(interpolated)
    return result
    
    
for _ in range(10000):
    data = [random() for _ in range(randint(2, 10000))]
    assert get_percentiles(data) == quantiles(data, n=100, method="inclusive")

[AdamISZ]

Great. I will check on a couple old Python versions as a sanity check. Since this has such a low/zero security impact, we don't need to worry too much (the tests are nice though, of course!).

[AdamISZ]

Edit: oh, well, sorry, I'm having trouble getting Joinmarket's install to work with either py37 or py36. Not sure it's worth putting any more time into it.

[PulpCattel]

I'm having trouble getting Joinmarket's install to work with either py37 or py36

@AdamISZ
I assume so, but just to be clear, the issues you are mentioning are unrelated to this PR/bond_calc.py, right?

[kristapsk]

I should get back to #1218, then CI would at least run test suite for Python 3.6.

EDIT: Sorry, I'm idiot, I added different Bitcoin Core versions, Python was there before.

[kristapsk]

IMO this can be merged. Test suite passes in CI, @PulpCattel has tested it and most people nowadays likely will have Python 3.8+ anyway.