@Tommy.Carstensen: Unfortunately, Apple has deprecated use of the system-supplied OpenSSL libraries and with the latest releases no longer supply the header files needed to build with them.  They are very old anyway.  So, you need to supply a version of them.  The easiest way is to get them from a third-party package manager like Homebrew or MacPorts but you certainly can download an OpenSSL source release from https://www.openssl.org and build the libraries themselves.  If you do not have administrator access, you will probably need to modify the OpenSSL build using at least --prefix to install to a non-system location and then rerun Python's ./configure with CFLAGS and LDFLAGS pointing to the installed location of your OpenSSL.