How to install the python library inside a virtualenv?

After following the instructions in the INSTALL.md file, I can successfully import `bcc` from the system python interpreter (v3.6). But I cannot import `bcc` from the python within the virtualenv (v3.7). I want to write BPF program with some 3.7 features (`time.time_ns`) How can I install the python bcc library inside a virtualenv? I'm used to installing per-virtualenv python packages using pip, but this library doesn't seem to have that option. Thanks in advance. :)

Start with INSTALL.md and compare its installation steps with the reported Python 3.6 system interpreter and Python 3.7 virtualenv. Verify whether the documented process covers installing and importing the Python bcc library in a virtualenv; done means the supported procedure and any limitations are clearly documented.

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