Import features from ssl module with more granularity

[sethmlarson]

Very subtle bug here that was caused by isort reordering the imports within try-except. Here's the diff of urllib3.util.ssl_ between 1.25.11 and 1.26.1 that caused the bug:

 try:  # Test for SSL features
     import ssl
-    from ssl import wrap_socket, CERT_REQUIRED
     from ssl import HAS_SNI  # Has SNI?
+    from ssl import CERT_REQUIRED, wrap_socket
+
+    from .ssltransport import SSLTransport
 except ImportError:
     pass

Notice how HAS_SNI is now being imported before wrap_socket and CERT_REQUIRED and HAS_SNI is a Python 2.7.9 feature. This meant that imports on Python 2.7.8 and earlier weren't getting wrap_socket imported in time before HAS_SNI was causing an ImportError.

Verified this locally with the python:2.7.8 Docker container, we currently don't have CI for this Python version.

Closes #2051

[codecov]

Codecov Report

Merging #2052 (260a7bd) into master (6611153) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2052   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines         2296      2298    +2     
=========================================
+ Hits          2296      2298    +2     

Impacted Files	Coverage Δ
src/urllib3/util/ssl_.py	100.00% <100.00%> (ø)

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[nateprewitt]

[yaodingyd]

appreciate the quick turnaround!