v3.0: The "handle numbers better" release

Previously, wordfreq would group all digit sequences of the same 'shape',
with length 2 or more, into a single token and return the frequency of that
token, which would be a vast overestimate.

Now it distributes the frequency over all numbers of that shape, with an
estimated distribution that allows for Benford's law (lower numbers are more
frequent) and a special frequency distribution for 4-digit numbers that look
like years (2010 is more frequent than 1020).

More changes related to digits:

• Functions such as iter_wordlist and top_n_list no longer return
  multi-digit numbers (they used to return them in their "smashed" form, such
  as "0000").

• lossy_tokenize no longer replaces digit sequences with 0s. That happens
  instead in a place that's internal to the word_frequency function, so we can
  look at the values of the digits before they're replaced.

Other changes:

• wordfreq is now developed using poetry as its package manager, and with
  pyproject.toml as the source of configuration instead of setup.py.

• The minimum version of Python supported is 3.7.

• Type information is exported using py.typed.