Add 'next' convenience for tbump

[blink1073]

Fixes #213

[codecov-commenter]

Codecov Report

Merging #221 (a3b96c5) into master (b6ade2e) will increase coverage by 0.14%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master     #221      +/-   ##
==========================================
+ Coverage   84.92%   85.06%   +0.14%     
==========================================
  Files          19       19              
  Lines        2288     2303      +15     
  Branches      288      292       +4     
==========================================
+ Hits         1943     1959      +16     
+ Misses        247      246       -1     
  Partials       98       98              

Impacted Files	Coverage Δ
jupyter_releaser/util.py	71.90% <88.88%> (+1.46%)	⬆️
jupyter_releaser/tests/test_functions.py	98.51% <100.00%> (+0.09%)	⬆️

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