Try to fix problems with running httpbin in the CI builds

Fix the last known working Flask version, the latest one results in the
errors like the following

Traceback (most recent call last):
  File "/usr/lib/python3.8/runpy.py", line 185, in _run_module_as_main
    mod_name, mod_spec, code = _get_module_details(mod_name, _Error)
  File "/usr/lib/python3.8/runpy.py", line 111, in _get_module_details
    __import__(pkg_name)
  File "/home/runner/.local/lib/python3.8/site-packages/httpbin/__init__.py", line 3, in <module>
    from .core import *
  File "/home/runner/.local/lib/python3.8/site-packages/httpbin/core.py", line 22, in <module>
    from werkzeug.wrappers import BaseResponse
ImportError: cannot import name 'BaseResponse' from 'werkzeug.wrappers' (/home/runner/.local/lib/python3.8/site-packages/werkzeug/wrappers/__init__.py)

build/tools/httpbin.sh

@@ -35,6 +35,12 @@ httpbin_launch() {
             blinker==1.4 brotlipy==0.7.0 cffi==1.14.5 click==7.0 decorator==4.4.2
             itsdangerous==1.1.0 pycparser==2.20 raven==6.10.0 werkzeug==0.16.1'
             ;;
+
+        *)
+            # Installing Flask 2.1.0 and its dependency Werkzeug 2.1.0 results
+            # in failures when trying to run httpbin, so stick to an older but
+            # working version.
+            pip_explicit_deps='Flask==2.0.3 Werkzeug==2.0.3'
     esac
 
     # Ensure that we have at least some version of pip and setuptools required