fix: Add support for pytest-mock spy method fixing the SignatureAdapter

[fgmacedo]

Fixes #391 .

Changed SignatureAdapter class to fix a bug where cls.from_callable(method) would return a Signature object instead of a SignatureAdapter if the method already had a Signature. Ensures that cls.from_callable(method) always returns a SignatureAdapter.

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0 Bugs
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No Coverage information
0.0% Duplication

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Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b936236) 100.00% compared to head (2f17db9) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #392 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 20 20 Lines 985 991 +6 Branches 163 165 +2 ========================================= + Hits 985 991 +6 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `100.00% <100.00%> (ø)` | | 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=Fernando+Macedo#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/fgmacedo/python-statemachine/pull/392?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fernando+Macedo) | Coverage Δ | | |---|---|---| | [statemachine/signature.py](https://app.codecov.io/gh/fgmacedo/python-statemachine/pull/392?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fernando+Macedo#diff-c3RhdGVtYWNoaW5lL3NpZ25hdHVyZS5weQ==) | `100.00% <100.00%> (ø)` | |

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