update dev-requirements pins

pin 3.6 and then 3.7+

dev-requirements.txt

@@ -1,10 +1,17 @@
 # Invocations for common project tasks
 invoke==1.6.0
 invocations==2.6.0
-pytest==4.4.2
-pytest-relaxed==1.1.5
+# Pinning has some disadvantages when testing across multiple Python versions
+# 3.6 specific pins
+pytest==4.4.2;python_version=='3.6'
+pytest-relaxed==1.1.5;python_version=='3.6'
 # pytest-xdist for test dir watching and the inv guard task
-pytest-xdist==1.28.0
+pytest-xdist==1.28.0;python_version=='3.6'
+# 3.7+ specific
+pytest==7.2.0;python_version>='3.7'
+pytest-xdist==3.1.0;python_version>='3.7'
+pytest-relaxed==2.0.0;python_version>='3.7'
+
 mock==2.0.0
 # Linting!
 flake8==3.8.3