[fix] fix attrs install in requirements.txt

• Remove wrong attr package (causing conflict with attrs package in Sakia)
• Replace it with the good one: attrs

requirements.txt

-aiohttp >= 3.3.2
-pylibscrypt >= 1.7.1
-libnacl >= 1.6.1
-base58 >= 1.0.0
-jsonschema >= 2.6.0
-pypeg2 >= 2.15.2
-attr >= 0.3.1
-pyaes >= 1.6.1
+aiohttp>=3.3.2
+pylibscrypt>=1.7.1
+libnacl>=1.6.1
+base58>=1.0.0
+jsonschema>=2.6.0
+pypeg2>=2.15.2
+attrs==19.3.0
+pyaes>=1.6.1
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