Update toxfile to pin all dependencies to locked versions

Stop-gap until I can figure out how to create a tox env from the lockfile
Add missing pytest-cov dev dependency
Fix py3.6 sqlite compatibility error with pathlib

tox.ini

@@ -5,18 +5,19 @@ isolated_build = true
 [testenv]
 description = Run the unit tests (pytest)
 deps =
-    pytest
-    pytest-cov
-    toml
-    passlib
-    marshmallow
+    marshmallow == 3.5.0
+    passlib == 1.7.2
+    peewee == 3.13.1
+    pytest == 5.3.5
+    pytest-cov == 2.8.1
+    toml == 0.10.0
 commands =
     pytest --cov={envsitepackagesdir}/keyosk --cov-config .coveragerc tests/ --cov-report term-missing
 
 [testenv:lint]
 description = Check code formatting against black and pylint
 deps =
-    pre-commit
+    pre-commit == 2.1.0
     pylint == 2.3.1
     astroid == 2.2.5
 commands =
@@ -26,22 +27,22 @@ commands =
 [testenv:typing]
 description = Check type annotations with mypy
 deps =
-    mypy
+    mypy == 0.761
 commands =
     mypy keyosk --ignore-missing-imports --no-strict-optional
 
 [testenv:security]
 description = Check security vulnerabilities (bandit)
 deps =
-    bandit
+    bandit == 1.6.2
 commands =
     bandit --recursive keyosk
 
 [testenv:docs]
 description = Build documentation to check RST syntax (sphinx)
 deps =
-    sphinx >= 1.7.5, < 2.0
-    sphinx-autodoc-typehints >= 1.3.0, < 2.0
+    sphinx == 2.4.2
+    sphinx-autodoc-typehints == 1.10.3
 whitelist_externals =
     rm
 commands =