Handle ImportError and OSError when importing ctypes (pypa#6543)

Non-dynamic executables can raise OSError when importing ctypes
because dlopen(NULL) is called on module import and dlopen()
won't work on non-dynamic executables.

This commit teaches the glibc version sniffing module to
handle a missing or not working ctypes module.

news/6543.bugfix

@@ -0,0 +1 @@
+Work around failure to import ctypes when resolving glibc version string.

src/pip/_internal/utils/glibc.py

@@ -1,11 +1,16 @@
 from __future__ import absolute_import
 
-import ctypes
 import re
 import warnings
 
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 
+# Work around https://bugs.python.org/issue37060.
+try:
+    import ctypes
+except (ImportError, OSError):
+    ctypes = None
+
 if MYPY_CHECK_RUNNING:
     from typing import Optional, Tuple
 
@@ -14,6 +19,9 @@ def glibc_version_string():
     # type: () -> Optional[str]
     "Returns glibc version string, or None if not using glibc."
 
+    if not ctypes:
+        return None
+
     # ctypes.CDLL(None) internally calls dlopen(NULL), and as the dlopen
     # manpage says, "If filename is NULL, then the returned handle is for the
     # main program". This way we can let the linker do the work to figure out