Add console interface: calculator loop, ASCII title and tests

MIL-003 Console interface.

- Add calculator() with recursive restart: y continues with the result,
  n starts a new calculation, q quits
- Show one ASCII title at start: a calculator with the lettering to
  its right
- Re-prompt on invalid numbers, operations and answers; restart on
  division by zero
- Add __main__.py for python -m calc and the calc console script
- Add interface tests with input() fed through monkeypatch

Closes #22
Closes #23
Closes #24
Closes #25

Task: MIL-003#1
Task: MIL-003#2
Task: MIL-003#3
Task: MIL-003#4

src/calc/calculator.py

@@ -0,0 +1,112 @@
+"""!
+@file calculator.py
+@brief The interactive console calculator.
+"""
+
+from calc.constants import (
+    ANSWER_CONTINUE,
+    ANSWER_NEW,
+    ANSWER_QUIT,
+    ASCII_TITLE,
+    MESSAGE_DIVIDE_BY_ZERO,
+    MESSAGE_GOODBYE,
+    MESSAGE_INVALID_ANSWER,
+    MESSAGE_INVALID_NUMBER,
+    MESSAGE_INVALID_OPERATION,
+    PROMPT_AGAIN,
+    PROMPT_FIRST_NUMBER,
+    PROMPT_NEXT_NUMBER,
+    PROMPT_OPERATION,
+)
+from calc.operations import OPERATIONS
+
+
+def format_number(value: float) -> str:
+    """!
+    @brief Format a number without a trailing ".0" for whole values.
+    @param value The number to format.
+    @return The text to show the user.
+    """
+    return str(int(value)) if value.is_integer() else str(value)
+
+
+def read_number(prompt: str) -> float:
+    """!
+    @brief Ask for a number until the user types a valid one.
+    @param prompt The text shown to the user.
+    @return The number typed by the user.
+    """
+    while True:
+        try:
+            return float(input(prompt))
+        except ValueError:
+            print(MESSAGE_INVALID_NUMBER)
+
+
+def read_operation() -> str:
+    """!
+    @brief Show the available operations and ask for one until it is valid.
+    @return The symbol of the chosen operation, a key of OPERATIONS.
+    """
+    for symbol in OPERATIONS:
+        print(symbol)
+    while True:
+        symbol = input(PROMPT_OPERATION).strip()
+        if symbol in OPERATIONS:
+            return symbol
+        print(MESSAGE_INVALID_OPERATION)
+
+
+def read_answer(result: float) -> str:
+    """!
+    @brief Ask whether to continue with the result, start anew or quit.
+    @param result The result of the last calculation.
+    @return One of ANSWER_CONTINUE, ANSWER_NEW or ANSWER_QUIT.
+    """
+    while True:
+        answer = input(PROMPT_AGAIN.format(result=format_number(result)))
+        answer = answer.strip().lower()
+        if answer in (ANSWER_CONTINUE, ANSWER_NEW, ANSWER_QUIT):
+            return answer
+        print(MESSAGE_INVALID_ANSWER)
+
+
+def calculator() -> None:
+    """!
+    @brief Run one calculation session.
+
+    The user may continue with the previous result. Starting a new
+    calculation calls this function again (recursion), which gives a clean
+    restart. A division by zero also restarts the calculator.
+    """
+    num1 = read_number(PROMPT_FIRST_NUMBER)
+    while True:
+        symbol = read_operation()
+        num2 = read_number(PROMPT_NEXT_NUMBER)
+        try:
+            result = OPERATIONS[symbol](num1, num2)
+        except ZeroDivisionError:
+            print(MESSAGE_DIVIDE_BY_ZERO)
+            calculator()
+            return
+        print(
+            f"{format_number(num1)} {symbol} {format_number(num2)} "
+            f"= {format_number(result)}"
+        )
+        answer = read_answer(result)
+        if answer == ANSWER_CONTINUE:
+            num1 = result
+        elif answer == ANSWER_NEW:
+            calculator()
+            return
+        else:
+            print(MESSAGE_GOODBYE)
+            return
+
+
+def main() -> None:
+    """!
+    @brief Program entry point: show the ASCII title and start the calculator.
+    """
+    print(ASCII_TITLE)
+    calculator()