Remove unused imports. Explicitly import a few deep imports.

lib/x509.py

@@ -22,10 +22,8 @@
 # ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 # CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
-from datetime import datetime
-import sys
 from . import util
-from .util import profiler, print_error, bh2u
+from .util import profiler, bh2u
 import ecdsa
 import hashlib