build: Use tox "constraints" and "deps" configuration options rather than assembling a custom install_command.

Author: seifertm

tox.ini

@@ -7,20 +7,16 @@ passenv =
 
 [testenv]
 extras = testing
-install_command = python -m pip install \
-    --requirement dependencies/default/requirements.txt \
-    --constraint dependencies/default/constraints.txt \
-    {opts} {packages}
+constraints = dependencies/default/constraints.txt
+deps = -r dependencies/default/requirements.txt
 commands = make test
 allowlist_externals =
     make
 
 [testenv:pytest-min]
 extras = testing
-install_command = python -m pip install \
-    --requirement dependencies/pytest-min/requirements.txt \
-    --constraint dependencies/pytest-min/constraints.txt \
-    {opts} {packages}
+constraints = dependencies/pytest-min/constraints.txt
+deps = -r dependencies/pytest-min/requirements.txt
 commands = make test
 allowlist_externals =
     make
@@ -29,9 +25,7 @@ allowlist_externals =
 allowlist_externals =
     git
 extras = docs
-deps =
-     --requirement dependencies/default/requirements-docs.txt
-     --constraint dependencies/default/constraints.txt
+deps = -r dependencies/default/requirements-docs.txt
 change_dir = docs
 description = Build The Docs with {basepython}
 commands =