textproc/py-readtime: New port

Calculates the time some text takes the average human to read, based on
Medium's read time forumula.

Medium's Help Center says:

"Read time is based on the average reading speed of an adult (roughly
265 WPM). We take the total word count of a post and translate it into
minutes, with an adjustment made for images. For posts in Chinese,
Japanese and Korean, it's a function of number of characters (500
characters/min) with an adjustment made for images."

Source: https://help.medium.com/hc/en-us/articles/214991667-Read-time