[build] Fixes a bug that prevented the pytest-min tox environment from correctly using the minimum pytest version.

The "deps" option does not specify the package dependencies, but a list of packages that should be installed before the package under test. However, it's possible ot use "install_command" to specify a constraints file.
see tox-dev/tox#1939 (comment)

Signed-off-by: Michael Seifert <m.seifert@digitalernachschub.de>

Author: seifertm

tox.ini

@@ -7,18 +7,20 @@ passenv =
 
 [testenv]
 extras = testing
-deps =
-     --requirement dependencies/default/requirements.txt
-     --constraint dependencies/default/constraints.txt
+install_command = python -m pip install \
+    --requirement dependencies/default/requirements.txt \
+    --constraint dependencies/default/constraints.txt \
+    {opts} {packages}
 commands = make test
 allowlist_externals =
     make
 
 [testenv:pytest-min]
 extras = testing
-deps =
-     --requirement dependencies/pytest-min/requirements.txt
-     --constraint dependencies/pytest-min/constraints.txt
+install_command = python -m pip install \
+    --requirement dependencies/pytest-min/requirements.txt \
+    --constraint dependencies/pytest-min/constraints.txt \
+    {opts} {packages}
 commands = make test
 allowlist_externals =
     make