title: ERROR Could not build wheels for xmlsec, which is required to install pyproject.toml-based projects
body: <p>I am having an error when trying to install Rasa X in local mode form Terminal. Using Python 3.7.4 and pip version 21.3.1. Using Mac.</p>
  <pre><code>  Building wheels for collected packages: xmlsec
    Building wheel for xmlsec (pyproject.toml) ... error
    ERROR: Command errored out with exit status 1:
    Complete output (14 lines):
    running bdist_wheel
    running build
    running build_py
    package init file 'src/xmlsec/__init__.py' not found (or not a regular file)
    creating build
    creating build/lib.macosx-10.9-x86_64-3.7
    creating build/lib.macosx-10.9-x86_64-3.7/xmlsec
    copying src/xmlsec/py.typed -&gt; build/lib.macosx-10.9-x86_64-3.7/xmlsec
    copying src/xmlsec/tree.pyi -&gt; build/lib.macosx-10.9-x86_64-3.7/xmlsec
    copying src/xmlsec/__init__.pyi -&gt; build/lib.macosx-10.9-x86_64-3.7/xmlsec
    copying src/xmlsec/constants.pyi -&gt; build/lib.macosx-10.9-x86_64-3.7/xmlsec
    copying src/xmlsec/template.pyi -&gt; build/lib.macosx-10.9-x86_64-3.7/xmlsec
    running build_ext
    error: Unable to invoke pkg-config.
    ----------------------------------------
    ERROR: Failed building wheel for xmlsec
  Failed to build xmlsec
  ERROR: Could not build wheels for xmlsec, which is required to install pyproject.toml-based projects
  </code></pre>
  <p>How can I solve this?</p>