Pass CFLAGS and LDFLAGS when running tests

tox.ini

@@ -17,6 +17,12 @@ skip_missing_interpreters = true
 [testenv]
 description = run the unit tests under {basepython}
 commands = python setup.py test -a "{posargs:-n auto}"
+# compilation flags can be useful when prebuilt wheels cannot be used, e.g.
+# PyPy 2 needs to compile the `cryptography` module. On macOS this can be done
+# by passing the following flags:
+# LDFLAGS="-L$(brew --prefix openssl@1.1)/lib"
+# CFLAGS="-I$(brew --prefix openssl@1.1)/include"
+passenv = LDFLAGS CFLAGS
 
 [testenv:fmt]
 description = run code formatting using black