Remove Python2 numeric relics

[eumiro]

• closes #xxxx
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Modernize the code to Python 3 by removing Python 2 numeric hacks, such as:

• a / b of two integers now always returns a float, so no need to convert a and/or b to float (or to use float literals)
• a // b of two integers returns an integer, use it instead of int(a / b)
• math.ceil, math.floor, and round return integer, so no need to convert the result to integer
• 1e6 is a float, converting it to int(1e6) is slower than 10 ** 6 (or 1_000_000)

[MarcoGorelli]

Nice, thanks @eumiro !

Did you use some tool to find these, or was it just inspection?

[eumiro]

Nice, thanks @eumiro !

Did you use some tool to find these, or was it just inspection?

Thank you. It was manual search&replace (with at least one typo as you could see, thank you, fixed, repushed), because every case is different. Usually it is a good idea to search for int(, float(, round, math., \de\d, \d\.0\b, etc.

[jreback]

thanks @eumiro very nice