fix: building a wheel

this fixes two issues when building a wheel using pyproject-build.
The first issue
```
ERROR Missing dependencies:
  poetry>=0.12
```
can be resolved by requiring poetry-core over poetry>=0.12

The second issue
```
ModuleNotFoundError: No module named 'poetry.masonry'
```
can be resolved by importing the correct module from poetry.core.

see also https://pypi.org/project/poetry-core/

pyproject.toml

@@ -20,8 +20,8 @@ mypy = "^0.790"
 pytest-cov = "^2.10.1"
 
 [build-system]
-requires = ["poetry>=0.12"]
-build-backend = "poetry.masonry.api"
+requires = ["poetry-core"]
+build-backend = "poetry.core.masonry.api"
 
 [tool.pytest.ini_options]
 addopts = "--strict-markers"