requirements.txt: use a newer flask version

This patch prevents this problem:

*** Operational MODE: single process ***
mounting api:app on /
Traceback (most recent call last):
  File "./api.py", line 17, in <module>
    from flask import Flask
  File "/usr/local/lib/python3.7/dist-packages/flask/__init__.py", line 19, in <module>
    from . import json
  File "/usr/local/lib/python3.7/dist-packages/flask/json/__init__.py", line 15, in <module>
    from itsdangerous import json as _json
ImportError: cannot import name 'json' from 'itsdangerous' (/usr/local/lib/python3.7/dist-packages/itsdangerous/__init__.py)
unable to load app 0 (mountpoint='/') (callable not found or import error)

See for reference:

* https://serverfault.com/questions/1094062/error-from-itsdangerous-import-json-as-json-importerror-cannot-import-name-j
* pallets/flask#4456

Signed-off-by: Arturo Borrero Gonzalez <[email protected]>
Change-Id: Ia5f4001a438b95d25c791f5efaa5170c16665547

requirements.txt

@@ -1,4 +1,6 @@
-Flask==1.1.2
+Flask==1.1.4
 Flask-RESTful==0.3.8
 PyYAML==5.3.1
 requests==2.25.1
+# transitory problem https://github.com/pallets/flask/issues/4456
+markupsafe==2.0.1