title: Can't install xmlsec using PIP command
body: <p><code>pip install xmlsec</code> commands throws the below error.</p>
  <pre><code>ERROR: Command errored out with exit status 1:
     command: /home/xxx/PycharmProjects/saml_impl/saml_impl/venv/bin/python /home/sathia/PycharmProjects/saml_impl/saml_impl/venv/lib/python3.8/site-packages/pip/_vendor/pep517/_in_process.py build_wheel /tmp/tmpu_b5m5vz
         cwd: /tmp/pip-install-gblz98sr/xmlsec
    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.linux-x86_64-3.8
    creating build/lib.linux-x86_64-3.8/xmlsec
    copying src/xmlsec/py.typed -&gt; build/lib.linux-x86_64-3.8/xmlsec
    copying src/xmlsec/template.pyi -&gt; build/lib.linux-x86_64-3.8/xmlsec
    copying src/xmlsec/constants.pyi -&gt; build/lib.linux-x86_64-3.8/xmlsec
    copying src/xmlsec/__init__.pyi -&gt; build/lib.linux-x86_64-3.8/xmlsec
    copying src/xmlsec/tree.pyi -&gt; build/lib.linux-x86_64-3.8/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 use PEP 517 and cannot be installed directly'
  </code></pre>
  <p>I don't know how to resolve this issue. I tried to install other xmlsec package too, nothing worked.</p>