Fix incompatibility with tox 2.5

Apparently on tox 2.5, `usedevelop` overrides `skip_install`, so we end up
trying to install the full dependencies even for the `-old` environment.

changelog.d/9769.misc

@@ -0,0 +1 @@
+Fix incompatibility with `tox` 2.5.

tox.ini

@@ -104,7 +104,8 @@ usedevelop=true
 # A test suite for the oldest supported versions of Python libraries, to catch
 # any uses of APIs not available in them.
 [testenv:py35-old]
-skip_install=True
+skip_install = true
+usedevelop = false
 deps =
     # Old automat version for Twisted
     Automat == 0.3.0
@@ -136,7 +137,8 @@ commands =
     python -m synmark {posargs:}
 
 [testenv:packaging]
-skip_install=True
+skip_install = true
+usedevelop = false
 deps =
     check-manifest
 commands =
@@ -154,7 +156,8 @@ extras = lint
 commands = isort -c --df --sp setup.cfg {[base]lint_targets}
 
 [testenv:check-newsfragment]
-skip_install = True
+skip_install = true
+usedevelop = false
 deps = towncrier>=18.6.0rc1
 commands =
    python -m towncrier.check --compare-with=origin/develop
@@ -163,7 +166,8 @@ commands =
 commands = {toxinidir}/scripts-dev/generate_sample_config --check
 
 [testenv:combine]
-skip_install = True
+skip_install = true
+usedevelop = false
 deps =
     coverage
     pip>=10 ; python_version >= '3.6'
@@ -173,14 +177,16 @@ commands=
     coverage report
 
 [testenv:cov-erase]
-skip_install = True
+skip_install = true
+usedevelop = false
 deps =
     coverage
 commands=
     coverage erase
 
 [testenv:cov-html]
-skip_install = True
+skip_install = true
+usedevelop = false
 deps =
     coverage
 commands=