Espeak mbrola backend

[hadware]

The Espeak backend couldn't output "standard" sampa considering Espeak's current API. Here's what this PR adds/changes :

• adds a new backend (EspeakMbrolaBackend). This backend is much simpler than the regular Espeak backend since it doesn't support words or language switching. It's only made to phonemize using mbrola voices.
• removed the former sampa mappings and the corresponding logic in the regular espeak backend
• added the new backend to the main script. This should probably be checked.
• fixed unittests to account for the new backend
• factorized the code into an abstract class between the two backends, since they share a lot of espeak-detection code.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@30d048f). Click here to learn what that means. The diff coverage is 99.08%.

@@            Coverage Diff            @@
##             master      #36   +/-   ##
=========================================
  Coverage          ?   99.67%           
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  Files             ?       14           
  Lines             ?      618           
  Branches          ?        0           
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  Hits              ?      616           
  Misses            ?        2           
  Partials          ?        0

Impacted Files	Coverage Δ
phonemizer/punctuation.py	100% <100%> (ø)
phonemizer/backend/__init__.py	100% <100%> (ø)
phonemizer/backend/base.py	100% <100%> (ø)
phonemizer/backend/segments.py	100% <100%> (ø)
phonemizer/main.py	100% <100%> (ø)
phonemizer/utils.py	100% <100%> (ø)
phonemizer/__init__.py	100% <100%> (ø)
phonemizer/backend/festival.py	100% <100%> (ø)
phonemizer/phonemize.py	100% <100%> (ø)
phonemizer/backend/espeak.py	98.75% <96.92%> (ø)

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