Specify pytest version in Makefile too

d875b58 added it to libweasyl[development], but libweasyl isn’t installed often enough in the Makefile to use that.

Makefile

@@ -44,7 +44,7 @@ $(VE): etc/requirements.txt
 	test -e $@ || { virtualenv $@; cp etc/pip.conf $@ ; \
                $@/bin/pip install -U pip setuptools -i https://pypi.python.org/simple ; }
 	$@/bin/pip install $(USE_WHEEL) -r etc/requirements.txt -e .
-	$@/bin/pip install $(USE_WHEEL) pytest flake8
+	$@/bin/pip install $(USE_WHEEL) pytest==4.6.5 flake8
 	touch $@
 
 .PHONY: install-libweasyl