Closes #261 "none" mode for key signature.

[manoskary]

Simple fix for mode=='none'.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (093168a) 85.65% compared to head (ec49da1) 85.65%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #262 +/- ## ======================================== Coverage 85.65% 85.65% ======================================== Files 75 75 Lines 12845 12845 ======================================== Hits 11003 11003 Misses 1842 1842 ``` | [Impacted Files](https://app.codecov.io/gh/CPJKU/partitura/pull/262?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CPJKU) | Coverage Δ | | |---|---|---| | [partitura/io/matchfile\_utils.py](https://app.codecov.io/gh/CPJKU/partitura/pull/262?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CPJKU#diff-cGFydGl0dXJhL2lvL21hdGNoZmlsZV91dGlscy5weQ==) | `97.45% <100.00%> (ø)` | | | [partitura/utils/music.py](https://app.codecov.io/gh/CPJKU/partitura/pull/262?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CPJKU#diff-cGFydGl0dXJhL3V0aWxzL211c2ljLnB5) | `74.79% <100.00%> (ø)` | |

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