Use InMemoryHistory instead of the (now abstract) History class.

This was the result of the backward incompatible change in
prompt-toolkit 0.47.

setup.py

@@ -39,7 +39,7 @@ setup(
     ] + [p for p, cond in conditional_dependencies.items() if cond],
     extras_require = {
         'ical': ['icalendar'],
-        'prompt-toolkit': ['prompt-toolkit >= 0.42'],
+        'prompt-toolkit': ['prompt-toolkit >= 0.47'],
         'edit-cmd-tests': ['mock'],
     },
     entry_points= {

topydo/cli/Prompt.py

@@ -22,7 +22,7 @@ import sys
 from topydo.cli.CLIApplicationBase import CLIApplicationBase, error, usage
 from topydo.cli.TopydoCompleter import TopydoCompleter
 from prompt_toolkit.shortcuts import get_input
-from prompt_toolkit.history import History
+from prompt_toolkit.history import InMemoryHistory
 
 from topydo.lib.Config import config, ConfigError
 
@@ -83,7 +83,7 @@ class PromptApplication(CLIApplicationBase):
 
     def run(self):
         """ Main entry function. """
-        history = History()
+        history = InMemoryHistory()
 
         while True:
             # (re)load the todo.txt file (only if it has been modified)