Deps: Pin tightly Selenium version

We need 4.0, but the recent releases always had some issues.

See SeleniumHQ/selenium#9292

Author: nijel

requirements-test.txt

@@ -1,4 +1,4 @@
 -r requirements-optional.txt
 coverage
 responses>=0.10.1,<0.13.0
-selenium>=4.0.0a6.post2,!=4.0.0.a7,!=4.0.0.b1
+selenium==4.0.0a6.post2