Fix unnecessary quoting in newick format

[mmore500]

Closes #200

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.69%. Comparing base (be79787) to head (017918a).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #201 +/- ## ========================================== - Coverage 88.73% 88.69% -0.05% ========================================== Files 114 114 Lines 12877 12813 -64 ========================================== - Hits 11427 11365 -62 + Misses 1450 1448 -2 ``` | [Flag](https://app.codecov.io/gh/jeetsukumaran/DendroPy/pull/201/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeet+Sukumaran) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/jeetsukumaran/DendroPy/pull/201/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeet+Sukumaran) | `88.69% <ø> (-0.05%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeet+Sukumaran#carryforward-flags-in-the-pull-request-comment) to find out more.

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