fix for importing pxr.Tf on windows anaconda 3.8+ interpreters

cpython changed the way the dlls are loaded on windows for python-3.8+.
USD implemented a workaround for this change, installed into
pxr.__init__ (and Tf.__init__).  However, anaconda python interpreters
were patched to behave more like pre-3.8:

   https://github.com/conda-forge/python-feedstock/blob/8195ba1178041b7461238e8c5680eee62f5ea9d0/recipe/patches/0020-Add-CondaEcosystemModifyDllSearchPath.patch#L37

So we now check for anaconda interpreters before using the 3.8+ only
fix.

Fixes issue PixarAnimationStudios#1602

BUILDING.md

@@ -603,10 +603,15 @@ platforms to avoid issues with Boost config files (introduced in Boost version
 to use Boost specified config files for their USD build, specify 
 -DBoost_NO_BOOST_CMAKE=OFF when running cmake.
 
-2. Windows and Python 3.8+
+2. Windows and Python 3.8+ (non-Anaconda)
 Python 3.8 and later on Windows will no longer search PATH for DLL dependencies.
 Instead, clients can call `os.add_dll_directory(p)` to set paths to search.
 By default on that platform USD will iterate over PATH and add all paths using
 `os.add_dll_directory()` when importing Python modules. Users may override
 this by setting the environment variable `PXR_USD_WINDOWS_DLL_PATH` to a PATH-like
 string. If this is set, USD will use these paths instead.
+
+Note that the above does not apply to Anaconda python 3.8+ interpreters, as they
+are modified to behave like pre-3.8 python interpreters, and so continue to use
+the PATH for DLL dependencies.  When running under Anaconda users should
+configure their system the same way they did for pre-python 3.8.

build_scripts/pypi/package_files/setup.py

@@ -74,8 +74,15 @@ def windows():
 dllPath = os.path.split(os.path.realpath(__file__))[0]
 if sys.version_info >= (3, 8, 0):
     os.environ['PXR_USD_WINDOWS_DLL_PATH'] = dllPath
-else:
-    os.environ['PATH'] = dllPath + os.pathsep + os.environ['PATH']
+# Note that we ALWAYS modify the PATH, even for python-3.8+. This is because:
+#    - Anaconda python interpreters are modified to use the old, pre-3.8, PATH-
+#      based method of loading dlls
+#    - extra calls to os.add_dll_directory won't hurt these anaconda
+#      interpreters
+#    - similarly, adding the extra PATH entry shouldn't hurt standard python
+#      interpreters
+#    - there's no canonical/bulletproof way to check for an anaconda interpreter
+os.environ['PATH'] = dllPath + os.pathsep + os.environ['PATH']
 ''')
 
 # Get the readme text