Prevent calendar out of screen. Fixes #100

[Wolf-SO]

If calendar goes out of screen on drop_down, adjust position. (@todo take taskbar/dock area into account)

[photodude]

Looks like python 3.6 and 3.7 builds failed. Not sure if those are related to your code change or something in the build.

%PYTHON%\python.exe -m nose --with-coverage

58......E.........

59======================================================================

60ERROR: test_calendar_textvariable (tests.test_calendar.TestCalendar)

61----------------------------------------------------------------------

62Traceback (most recent call last):

63 File "C:\projects\tkcalendar\tests\test_calendar.py", line 228, in test_calendar_textvariable

64 year=2015, month=1, day=3, textvariable=var)

65 File "C:\projects\tkcalendar\tkcalendar\calendar_.py", line 268, in __init__

66 self._sel_date = parse_date(self._textvariable.get(), locale)

67 File "C:\PYTHON36\lib\site-packages\babel\dates.py", line 1201, in parse_date

68 raise ParseError("No numbers were found in input")

69babel.dates.ParseError: No numbers were found in input

[Wolf-SO]

@photodude I'm not sure either, but from my perspective it has nothing to do with the changes in my PR.

[photodude]

I agree, I'm starting to think it may be something only in the build checks as the other open PRs also look like they have this same issue.

[photodude]

@Wolf-SO Looks like there is some issue with Babel. The unit testing framework is also unmaintained #107 Which between babel and nose failures it prevents testing with current versions of python see #106

I'm starting to feel like @j4321 has moved on and is no longer maintaining this library. I'm searching for an alternative but I'm not finding any good alternatives.