Translate library/string.po rst, 2, 19, 26 (#677)

* translate: rst: 2, 19, 26

* fix: change some terms

* fix: change the translation of constant

* Apply suggestions from code review

---------

Co-authored-by: Wei-Hsiang (Matt) Wang <mattwang44@gmail.com>

Author: Bradley-Tsai

library/string.po

@@ -21,7 +21,7 @@ msgstr ""
 
 #: ../../library/string.rst:2
 msgid ":mod:`string` --- Common string operations"
-msgstr ""
+msgstr ":mod:`string` --- 常見的字串操作"
 
 #: ../../library/string.rst:7
 msgid "**Source code:** :source:`Lib/string.py`"
@@ -37,7 +37,7 @@ msgstr ":ref:`string-methods`"
 
 #: ../../library/string.rst:19
 msgid "String constants"
-msgstr ""
+msgstr "字串常數"
 
 #: ../../library/string.rst:21
 msgid "The constants defined in this module are:"
@@ -49,6 +49,7 @@ msgid ""
 "`ascii_uppercase` constants described below.  This value is not locale-"
 "dependent."
 msgstr ""
+"下文描述的 :const:`ascii_lowercase` 和 :const:`ascii_uppercase` 常數的串接，該值不依賴於區域設定。"
 
 #: ../../library/string.rst:32
 msgid ""