Support running with CPython interpreter's -OO flag

[jcharlong]

My application runs fine with CPython when not supplying any optimization flags and with the -O flag, but fails when -OO is supplied. -OO removes docstrings, & the following error occurs in my application when using -OO:

  File "/var/volatile/tmp/.mount_hl1001bZZXj5/usr/lib/python3/dist-packages/fs/opener/registry.py", line 216, in open_fs
    from ..base import FS
  File "/var/volatile/tmp/.mount_hl1001bZZXj5/usr/lib/python3/dist-packages/fs/base.py", line 94, in <module>
    class FS(object):
  File "/var/volatile/tmp/.mount_hl1001bZZXj5/usr/lib/python3/dist-packages/fs/base.py", line 607, in FS
    getbytes = _new_name(readbytes, "getbytes")
  File "/var/volatile/tmp/.mount_hl1001bZZXj5/usr/lib/python3/dist-packages/fs/base.py", line 88, in _new_name
    _method.__doc__ += deprecated_msg
TypeError: unsupported operand type(s) for +=: 'NoneType' and 'str'

Created a simple PR which fixes the issue for my application in #414 .

[althonos]

Closed by #414.