If pip install -r requirements.txt
in your virtualenv gives an error like:
OpenSSL/crypto/x509.h:17:25: fatal error: openssl/ssl.h: No such file or directory
#include <openssl/ssl.h>
^
compilation terminated.
resulting in the final error
error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
Make sure you have libssl-dev installed from your system's package manager:
$ sudo apt-get install libssl-dev