Inject $TOX_REQUIRES to tox config

E.g. use TOX_REQUIRES="virtualenv<20.22.0" to test tox provision for old Pythons.

smoke/venv.fmf

@@ -35,6 +35,7 @@ environment:
     VERSION: 3.7
     METHOD: venv
     TOX: true
+    TOX_REQUIRES: ""
 duration: 10m
 component+:
     - python2

smoke/venv.sh

@@ -8,6 +8,7 @@ VERSION=${VERSION:-3.8}
 PYTHON=${PYTHON:-python$VERSION}
 METHOD=${METHOD:-venv}
 TOX=${TOX:-true}
+TOX_REQUIRES=${TOX_REQUIRES:-}
 CYTHON=${CYTHON:-true}
 INSTALL_OR_SKIP=${INSTALL_OR_SKIP:-false}
 
@@ -109,8 +110,11 @@ else
     export TOXENV=$PYTHON
 fi
 
-cat > tox.ini << EOF
-[tox]
+echo '[tox]' > tox.ini
+if [ -n "$TOX_REQUIRES" ]; then
+    echo 'requires = '"$TOX_REQUIRES" >> tox.ini
+fi
+cat >> tox.ini << EOF
 skipsdist = True
 [testenv]
 commands = python -c 'import sys; print(sys.version)'
@@ -120,8 +124,11 @@ EOF
 tox | tee toxlog
 grep "^$VERSION\.[0-9]" toxlog
 
-cat > tox.ini << EOF
-[tox]
+echo '[tox]' > tox.ini
+if [ -n "$TOX_REQUIRES" ]; then
+    echo 'requires = '"$TOX_REQUIRES" >> tox.ini
+fi
+cat >> tox.ini << EOF
 skipsdist = True
 [testenv]
 deps = pytest

tests.yml

@@ -20,13 +20,13 @@
         run: runuser testuser -c './venv.sh'
     - smoke27:
         dir: smoke
-        run: VERSION=2.7 INSTALL_OR_SKIP=true METHOD=virtualenv ./venv.sh
+        run: VERSION=2.7 INSTALL_OR_SKIP=true METHOD=virtualenv TOX_REQUIRES="virtualenv<20.22.0" ./venv.sh
     - smoke27_seeder_pip:
         dir: smoke
-        run: VERSION=2.7 INSTALL_OR_SKIP=true METHOD=virtualenv-seeder-pip ./venv.sh
+        run: VERSION=2.7 INSTALL_OR_SKIP=true METHOD=virtualenv-seeder-pip TOX=false ./venv.sh
     - smoke36:
         dir: smoke
-        run: runuser testuser -c 'VERSION=3.6 METHOD=venv TOX=false ./venv.sh'
+        run: runuser testuser -c 'VERSION=3.6 METHOD=venv TOX_REQUIRES="virtualenv<20.22.0" ./venv.sh'
     - smoke37:
         dir: smoke
         run: VERSION=3.7 METHOD=venv INSTALL_OR_SKIP=true ./venv.sh