docs: compat with older tox, pin sphinx deps

+ Use `whitelist_externals` to be compatible with
  older tox versions

+ Pin sphinx deps to versions that work, the floating
  dependencies are currently producing a broken build

Author: Jason Mobarak

generator/requirements.txt

@@ -1,18 +1,18 @@
-Jinja2>3
-pyyaml
-coverage
-docutils
-flake8
-mccabe
-numpy
-pybase64
-python-rapidjson
-pep8
-pyflakes
-pylint
-pytest
-tox
-voluptuous
-pytest-cov
-pytest-xdist
-dataclasses
+coverage~=6.0
+dataclasses~=0.6
+docutils==0.17.*
+flake8~=4.0
+Jinja2~=3.0
+mccabe~=0.6
+numpy~=1.21
+pybase64~=1.2
+python-rapidjson~=1.5
+pep8~=1.7
+pyflakes~=2.4
+pylint~=2.11
+pytest~=6.2
+pytest-cov~=3.0
+pytest-xdist~=2.4
+pyyaml            # PyYAML API is very stable
+tox               # Tox is (generally) designed to be very backwards compatible
+voluptuous~=0.12

tox.ini

@@ -5,7 +5,7 @@ skipsdist = True
 envlist = py
 
 [testenv]
-allowlist_externals =
+whitelist_externals =
   make
 deps = -rrequirements.txt
 commands = python -m sbpg.generator {posargs}