Add pep next command to find the next available number

[hugovk]

It checks the published PEPs at https://peps.python.org/api/peps.json, and open PRs beginning "PEP xxx: " at https://github.com/python/peps/pulls, combines them, then goes through them to find the first available number.

And we start at 400, because there's some unassigned numbers below this:

• 14 -> 19
• 21 -> 41
• 43 -> 99
• 104 -> 159
• 161 -> 199
• 300
• 388

For example:

❯ pep next
Next available PEP: 730

[codecov]

Codecov Report

Merging #48 (ff9f5bc) into main (3ba404a) will increase coverage by 3.07%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main       #48      +/-   ##
===========================================
+ Coverage   96.92%   100.00%   +3.07%     
===========================================
  Files           4         4              
  Lines         195       230      +35     
===========================================
+ Hits          189       230      +41     
+ Misses          6         0       -6     

Flag	Coverage Δ
macos-latest	100.00% <100.00%> (+3.07%)	⬆️
ubuntu-latest	100.00% <100.00%> (+3.07%)	⬆️
windows-latest	100.00% <100.00%> (+3.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/pepotron/__init__.py	100.00% <100.00%> (+5.55%)	⬆️
tests/test_pepotron.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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