setup: set explicit minimum version for all dependencies

Kind of arbitrarily chosen, but it seems like reasonable not-too-recent
versions that still provide a consistent baseline.

dev_requirements.txt

@@ -2,7 +2,7 b' pytest >= 4.6.6, < 4.7'
 pytest-sugar >= 0.9.2, < 0.10
 pytest-benchmark >= 3.2.2, < 3.3
 pytest-localserver >= 0.5.0, < 0.6
-mock < 3.1
-Sphinx < 1.9
-WebTest < 2.1
+mock >= 3.0.0, < 3.1
+Sphinx >= 1.8.0, < 1.9
+WebTest >= 2.0.3, < 2.1
 isort == 4.3.21

setup.py

@@ -41,7 +41,7 b" is_windows = __platform__ in ['Windows']"
 
 requirements = [
     "alembic >= 0.8.0, < 1.1",
-    "gearbox < 1",
+    "gearbox >= 0.1.0, < 1",
     "waitress >= 0.8.8, < 1.4",
     "WebOb >= 1.7, < 1.9",
     "backlash >= 0.1.2, < 1",