ENH: use constant f32 eps, not np.finfo() during import

NumPy docs for `np.finfo()` say not to call it during import (at module scope).
It's a relatively expensive call, and it modifies the GIL state.
Now we just hard-code it, because it is always the value anyway.
This avoids touching the GIL at import, which helps avoid deadlocks in practice.

Closes pandas-dev#14641.

pandas/core/indexing.py

@@ -1852,7 +1852,7 @@ def _convert_key(self, key, is_setter=False):
 
 
 # 32-bit floating point machine epsilon
-_eps = np.finfo('f4').eps
+_eps = 1.1920929e-07
 
 
 def length_of_indexer(indexer, target=None):