update image link in README

README.md

 # A tool to manage certificates lifecycle on Vault, Redis, Consul
 
-![Wile Coyote](https://filesender.geant.org/images/wile_coyote.jpg "Wile coyote")
+![Wile Coyote](https://cds.geant.org/images/wile_coyote.jpg "Wile coyote")
 
 ## Install the package
 

wile_coyote/bin/cert2json

@@ -38,8 +38,9 @@ else:
 
 
 def inspect_certificate(cert):
-    """ extract data from certificate """
-    pem_data = open(os.path.join(cert, 'fullchain.pem'), "rb").read()
+    """ extract data from the certificate """
+    with open(os.path.join(cert, 'fullchain.pem'), "rb") as pem_file:
+        pem_data = pem_file.read()
     cert = x509.load_pem_x509_certificate(pem_data, default_backend())
     cert_name = cert.subject.get_attributes_for_oid(x509.OID_COMMON_NAME)[0].value.strip()
     serial_raw = f'{cert.serial_number:x}'.upper()