That's a very dangerous trick and I advise against it. You are modifying the global linker path and inject custom OpenSSL libraries into it. This may affect and disrupt other programs or OS core tools.

Instead compile the _ssl and _hashlib module with rpath, e.g. LD_RUN_PATH. You also don't have to modify CFLAGS or LDFLAGS. --with-openssl does that for you.

$ export LD_RUN_PATH=/home/heimes/dev/python/multissl/openssl/1.1.1f/lib
$ ./configure --with-openssl=/home/heimes/dev/python/multissl/openssl/1.1.1f -C
$ make
$ unset LD_RUN_PATH
$ ldd build/lib.linux-x86_64-3.9/_ssl.cpython-39-x86_64-linux-gnu.so
        linux-vdso.so.1 (0x00007ffc124eb000)
        libssl.so.1.1 => /home/heimes/dev/python/multissl/openssl/1.1.1f/lib/libssl.so.1.1 (0x00007fd3d7cab000)
        libcrypto.so.1.1 => /home/heimes/dev/python/multissl/openssl/1.1.1f/lib/libcrypto.so.1.1 (0x00007fd3d7974000)
        libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fd3d791c000)
        libc.so.6 => /lib64/libc.so.6 (0x00007fd3d7753000)
        libdl.so.2 => /lib64/libdl.so.2 (0x00007fd3d774c000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fd3d7d8e000)