Virtualenv-api

* 46: Blacklist env var in subprocess calls to fix bug in MacOS/Homebrew installs (irvinlim)
* Change build versions to match supported Python versions (sjkingo)

* 40: Fix for test cases failing (irvinlim)
* 39: Fix for `is_installed` not handling capitilized names properly (irvinlim)
* Support Python 3.7 (sjkingo)

* Fix bug in test script where temporary directories were not being removed (sjkingo)
* 35: Don't pass unsupported argument to old versions of pip (sjkingo)
* Always call pip through the Python interpreter to avoid a too-long shebang error (sjkingo)
* 26: Fix incorrect subclassing of base test class that was causing extreme test durations (sjkingo)

* 34: Add support for `pip -r` requirements files (jzafran and sjkingo)

* 33: Add support for `system-site-packages` option (evansde77)

* 31: Workaround to prevent shebang length errors when calling pip (rmb938)