Add some examples about standard library.

Author: hermite2053

standard_lib/math_module/main.py

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+# math module example
+
+import math
+print(f'pi = {math.pi}')
+print(f'cos(pi / 4) = {math.cos(math.pi / 4)}')

standard_lib/os_module/main.py

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+import os
+print(os.getcwd())  # current working directory
+os.system('echo hello')  # execute given command in a subshell.

standard_lib/sys_module/main.py

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+# Enter '$ python main.py foo bar'to execute this example script.
+# ['main.py', 'foo', 'bar'] should be outputed.
+import sys
+print(sys.argv)