Use mypy recommended conditional imports for compats (#154)

Concerning conditional imports based on the actual Python version at
runtime, Mypy recommendation is to explicitly use `sys.version_info` to
trigger the relevant import depending on the Python version.

Reference to the documentation:
https://mypy.readthedocs.io/en/stable/common_issues.html#python-version-and-system-platform-checks

Apply this approach to the `compat` module.

As a consequence, Mypy does an educated guess on the imports, and
`#type: ignore` inline comment is not needed anymore.

src/pyotp/compat.py

@@ -1,7 +1,9 @@
+import sys
+
 # Use secrets module if available (Python version >= 3.6) per PEP 506
-try:
-    from secrets import SystemRandom  # type: ignore
-except ImportError:
+if sys.version_info >= (3, 6):
+    from secrets import SystemRandom
+else:
     from random import SystemRandom
 
 random = SystemRandom()