Do not install code to build release notes

... because we require only doc requirements.

Also remove explicit usage of requirements.txt because it is now
automatically detected and installed.

Change-Id: I3a34df059ba84c409b2949a037fb905ee88c3fbb
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -12,13 +12,11 @@ constraints =
   {env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
 deps =
   -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt
 commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
 description =
   Run style checks.
-skip_install = true
 deps =
   pre-commit
   {[testenv:mypy]deps}
@@ -63,6 +61,7 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:releasenotes]
+skip_install = true
 deps =
   -r{toxinidir}/doc/requirements.txt
 allowlist_externals =