Add SupportsIndex support to sequence formatters

[avylove]

Mypy has been complaining about Sequence methods ljust(), rjust(), and center() which are part of str, but overwritten. It looks like these use the SupportsIndex type rather than int, which just means they have an __index__() method. Most of the time this means int, but a custom type could implement the method and then you could use it instead of an int.

>>> class Foo:
...     def __index__(self):
...         return 10
... 
>>> 'abcd'.rjust(Foo())
'      abcd'

This PR fixes those type annotations and adds parity support to the Sequence formatting methods (rjust(), ljust(), center(), truncate()), so they can also take a custom class that implements the __index__() method for width.

[codecov]

Codecov Report

Merging #234 (14644ea) into master (1e53577) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #234   +/-   ##
=======================================
  Coverage   95.36%   95.36%           
=======================================
  Files           9        9           
  Lines        1014     1015    +1     
  Branches      176      176           
=======================================
+ Hits          967      968    +1     
  Misses         43       43           
  Partials        4        4           

Impacted Files	Coverage Δ
blessed/sequences.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1e53577...14644ea. Read the comment docs.

[jquast]

very cool, thanks, and happy new year!