Prettify list encoding

[jcbrockschmidt]

More closely match official TOML examples for arrays by excluding trailing commas.

For example, the return value for
print(toml.dumps({"data": [1, 2, 3, "abc"]}))
that used to be
data = [ 1, 2, 3, "abc",]
will now be
data = [ 1, 2, 3, "abc" ]

Change also makes dump_list more Pythonic.

[codecov-io]

Codecov Report

Merging #349 (99d17a2) into master (3f637db) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #349      +/-   ##
==========================================
- Coverage   89.74%   89.73%   -0.01%     
==========================================
  Files           5        5              
  Lines        1063     1062       -1     
==========================================
- Hits          954      953       -1     
  Misses        109      109              

Impacted Files	Coverage Δ
toml/encoder.py	88.26% <100.00%> (-0.06%)	⬇️

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