[EN-1370] cython added to extras as package should be compiled before

docs creation

Author: iamishalkin

pyproject.toml

@@ -37,6 +37,7 @@ pandas = "^0.25.1"
 # Refer to: https://github.com/sdispater/poetry/issues/129
 # Issue to watch for: https://github.com/python-poetry/poetry/issues/1644
 toml = { version = "^0.10.0", optional = true }
+cython = { version = "^0.29.13", optional = true }
 
 [tool.poetry.dev-dependencies]
 cython = "^0.29.13"
@@ -47,7 +48,7 @@ toml = "^0.10.0"
 sphinx_rtd_theme = "^0.4.3"
 
 [tool.poetry.extras]
-docs = ["toml"]
+docs = ["toml", "cython"]
 
 [tool.taskipy.tasks]
 clear = "find dxfeed/core -type f \\( -iname \\*.c -o -iname \\*.cpp -o -iname \\*.pyd -o -iname \\*.so \\) -delete"