Bump pytest max version to 8

We have a constraint that limits the maximum supported version of pytest
to the current major version, under the assumption that a new major
version release may break backwards compatibility. Now that pytest 7 has
been tested and is known to work with pytest-localserver, we can relax
the constraint to mark it as supported.

Author: diazona

tox.ini

@@ -9,9 +9,13 @@ downloadcache = {toxworkdir}/_download
 [testenv]
 description = run test suite under {basepython}
 deps =
-    pytest >=4, <7; python_version<'3.10'
-    pytest >=6.2.5, <7; python_version>='3.10'
-#    pytest-cov >=2, <3
+    # Future major versions of pytest may break backwards compatibility in a way
+    # that prevents them from working with this package, so we prevent testing
+    # with major versions that haven't been released yet. Each time a new major
+    # version of pytest is released, once it's confirmed to work with this
+    # package, we should bump this limit to the next major version.
+    pytest >=4, <8; python_version<'3.10'
+    pytest >=6.2.5, <8; python_version>='3.10'
     pytest-cov
     six
     requests