Add static calendar() method to jdatetime.date for displaying monthly calendar

[Peyman20122]

Hello dear engineer

Summary

This PR adds a new static method calendar() to the jdatetime.date class. It displays a monthly calendar (week-based table) for any given Shamsi (Jalali) year and month, similar to the Unix cal command or Python's calendar module.

Motivation

Currently, jdatetime provides date conversion and formatting but lacks a built‑in way to visualize a whole month as a weekly grid. This feature is useful for many Persian applications (CLI tools, scheduling widgets, etc.) and reduces boilerplate code for users.

Changes

• Added @staticmethod calendar(year=None, month=None, locale=None) to jdatetime.date.
• If year and month are omitted, it shows the current month.
• The method respects the thread‑local locale (Persian/English) for month names and weekday headers.
• Returns a multi‑line string (not printed directly) for flexibility.
• Handles leap years correctly (Esfand can have 30 days).
• Input validation (year range, month range) similar to the constructor.

Example usage

import jdatetime

# Current month calendar
print(jdatetime.date.calendar())

# Specific month (English)
print(jdatetime.date.calendar(1405, 2, locale='en_US'))

# Persian calendar
jdatetime.set_locale('fa_IR')
print(jdatetime.date.calendar(1405, 2))

Output example (English)

   Ordibehesht 1405
Sa Su Mo Tu We Th Fr
    1  2  3  4  5  6
 7  8  9 10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31

Additional notes

· The weekday headers assume Saturday = 0 (week starts on Saturday), consistent with jdatetime.date.weekday().
· The method does not depend on external libraries (e.g., numpy, hijridate).
· I have tested the method with various inputs and locales.

[Peyman20122]

Hello, good day, dear engineer
The problem with the code was that I used unittest.main() but did not import unittest.

[slashmili]

Hi @Peyman20122

Thanks for your work. Before looking into the PR, I want to mention that this module strives to provide interfaces similar to those provided by the standard library datetime provides. I only found a module named Calendar

My suggestion is that you work on jcalendar module in a separate repo and I'll be happy to support you on that!

[Peyman20122]

Hello dear engineer @slashmili

Thank you for your support

I told you the truth, maybe the user wants the calendar to be inside the jdatetime library, that's why I made the pull request

I also wrote a repository for the calendar, the link to which you can see below, it has both the Jalali calendar and the calendar with the moon in Scorpio
Thank you for your opinion about it

https://github.com/Peyman20122/Jalali-Calendar-library

[slashmili]

@Peyman20122 I'd rather follow the Unix philosophy, especially for such small libraries.

One more piece of advice: there are many Shamsi date conversion libraries in Python. The reason jdatetime has gained traction is that it serves as a drop-in replacement for Python's standard datetime library. I'd suggest taking the same approach and releasing a small jcalendar library.