Add default encoding as UTF-8

[thecodelearner]

Default encoding on Windows systems is cp1252
encoding="utf-8" fixes the issue with UnicodeDecodeError when mailmerge_database.csv contains Unicode characters in localized languages.
Thanks to @seshrs and @awdeorio for guiding me through this 😀

[codecov-commenter]

Codecov Report

Merging #98 into develop will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop      #98   +/-   ##
========================================
  Coverage    99.03%   99.03%           
========================================
  Files            6        6           
  Lines          311      311           
========================================
  Hits           308      308           
  Misses           3        3           

Impacted Files	Coverage Δ
mailmerge/__main__.py	99.21% <100.00%> (ø)

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[awdeorio]

Thanks for the PR @thecodelearner ! I don't have access to a windows machine. Could you confirm that the regression tests pass on your machine on this modified branch?

$ pytest

[thecodelearner]

one issue:
import sh under mailmerge/tests/test_main.py throws an error since, well, sh is unix only and throws the error "cannot import sh" on windows systems.

log: #97 (comment)

should I also do a neat little raise ImportError on this pr for that?

[thecodelearner]

No I don't think raising the error would do anything since all the tests are output to the bash shell with the sh() wrapper.
How do we go around with this?

[awdeorio]

Ah, this isn't going to work on Windows. The modern alternative to sh is subprocess.run(). Unfortunately, that function is only supported in recent versions of Python.

I tinkered with running a sanity check on Windows via Travis, but it looks like Python isn't supported on the Travis windows environment.

This is an uncomplicated change, so I'll just merge it in the absence of a way to write a decent unit test.