Update linking to correct CMake target

[marcosbento]

Since ecflow is built as a module (i.e. a shared library, dynamically loaded by the Python interpreter), the linkage in macOs requires passing flags -undefined and -dynamic_lookup. This is handled directly by CMake which, since CMake 3.15, defines a specific target Python3::Module to be used when building Python modules.

Re ECFLOW-1950

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.54%. Comparing base (80f8405) to head (1d9e56b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #98 +/- ## ======================================== Coverage 84.54% 84.54% ======================================== Files 606 606 Lines 54037 54037 ======================================== Hits 45686 45686 Misses 8351 8351 ```

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